Soybean sudden death syndrome resistant soybeans, soybean cyst nematode resistant soybeans and method of breeding and identifying resistant plants

ABSTRACT

The invention provides a method of introgressing soybean sudden death syndrome (SDS) or soybean cyst nematode (SCN) resistance into non-resistant soybean germplasm. Loci associated with SDS resistance or with SCN resistance in soybean lines known to be resistant to SDS or to SCN are used in marker assisted selection during introgression of SDS or SCN resistance into elite germplasm. In addition, the method may be used to confirm selection of resistance in new soybean cultivars.

PRIORITY APPLICATION INFORMATION

This application is a regular United States patent application under 37 C.F.R. §1.111(a) based on and claiming priority to U.S. Provisional Application Ser. No. 60/035,335 filed Jan. 14, 1997.

TECHNICAL FIELD

The present invention relates to soybeans and methods of soybean breeding. More particularly, the invention relates to soybean sudden death syndrome resistant soybean lines and methods of breeding same, the method involving genetic marker analysis; and soybean cyst nematode resistant soybean line and methods of breeding same, the methods involving genetic marker analysis.

The publications and other materials used herein to illuminate the background of the invention, and in particular cases, to provide additional details respecting the practice, are incorporated herein by reference, and for convenience, are referenced by author and date in the following text, and respectively group in the appended list of references.

BACKGROUND OF THE INVENTION

Soybeans are a major cash crop and investment commodity in North America and elsewhere. Soybean oil is one of the most widely used edible oils., and soybeans are used worldwide both in animal feed and in human food production.

Soybean sudden death syndrome (SDS) is a fungal disease of soybean (Glycine max (L.) Merr.), caused by Fusarium solani (Mart.) Sacc. f. sp. phaseoli (Burk.) Snyd. & Hans., type A (Rupe, 1989; Roy et al., 1989, O'Donnell and Gray, 1995). Since its discovery SDS has become one of the most destructive pests in soybean. It has been reported in nearly all states that soybean are grown, and it causes production problems in several states, being particularly destructive in Midwestern states. See generally (Mulrooney 1988, Gibson et al., 1994, Hartman et al., 1995, Wrather et al., 1995, 1996). For example, susceptible soybean cultivars had 5-60% lower yield than did resistant cultivars on F. solani infested sites in Illinois (Gibson et al., 1994).

Although the use of fungicides is effective in reducing the population level of the fungus, fungicide use is both uneconomical and environmentally unsound as a control measure in soybean production. Neither is crop rotation a practical means of fungal control since rotation with a non-susceptible crop for at least two years is necessary for reducing soybean losses. Therefore, it has long been felt by soybean breeders that use of resistant varieties is the most practical control measure.

Screening of soybean germplasm for resistance to SDS was begun soon after the discovery of the fungus in the U.S. (Gibson et al., 1994; Rupe et al., 1991). Soybean breeding for resistance to SDS has mostly utilized genes from the cultivar ‘Forrest’ (Hartwig 1970) and ‘Pyramid’ (Myers et al., 1980). However, low yield of these cultivars necessitates the introgression of their SDS resistance into elite germplasm with a minimum of linkage drag.

Resistance to SDS is multigenic and quantitative in soybean (Hnetkovsky et al., 1996; Njiti et al., 1996). Chang et al., (1996, 1997) estimated that Forrest has 5 genes required for resistance to SDS. Njiti et al., (1996) and Kilo et al., (1996) estimated that Pyramid has genes required for resistance to SDS, 2 that were different from those in Forrest. The multiple genes and genetic backgrounds involved contribute to the difficulty breeders have in developing SDS resistant soybean varieties.

Breeding programs for SDS resistance rely primarily on field evaluations where fungal populations occur. However, these populations can be mixtures of undetermined amplitypes (Achenbach et al., 1996) and the environment can vary thereby affecting the overwintering and infection capability of the fungus. Although evaluations using single oospore based fungal isolates in controlled greenhouse environments are possible, they are prohibitively expensive and are difficult to manage for larger breeding programs. These deficiencies in each evaluation method made SDS a difficult trait for soybean improvement.

Genetic markers closely linked to important genes may be used to indirectly select for favorable alleles more efficiently than direct phenotypic selection (Lander and Thompson 1990). SDS resistance loci have been associated with RFLP and microsatellite markers and tentatively mapped to linkage groups G (two), N and C1 (from Forrest); to linkage group C2 (from Essex); to linkage groups A2, B and G (from Pyramid) (Hnetkovsky et al., 1996; Chang et al., 1996, 1997; Abu-Thredeih et al., 1996; Kilo et al., 1996; Torto et al., 1996). In addition SDS resistance loci have been associated with the SCN resistance phenotype (Gibson et al., 1994) and soybean cyst nematode (SCN) resistance loci rhg1, rhg3 and Rhg4 on linkage groups G, B, and A2 respectively (Webb et al., 1995; Chang et al., 1996, 1997; Abu-Thredeih et al., 1996; Kilo et al., 1996).

U.S. Pat. No. 5,491,081, issued to Webb with Assignee Pioneer HiBred International, Inc. describes a method for introgressing SCN resistance into soybean germplasm as well as quantitative trait loci associated with SCN. It also describes SCN as a particular problem to soybean breeders and farmers. However, this patent does not discuss soybean SDS.

Therefore, it is of particular importance, both to the soybean breeders and to farmers who grow and sell soybeans as a cash crop, to identify, through genetic mapping, the quantitative trait loci (QTL) for resistance to SDS, and to identify additional QTL for resistance to SCN. Knowing the QTLs associated with resistance to SDS and to SCN, soybean breeders will be better able to breed SOS resistant and SCN resistant soybean that also possess other genotypic and phenotypic characteristics required for commercial soybean lines.

DISCLOSURE OF THE INVENTION

The invention provides a method of introgressing SDS and SCN resistance into non-resistant soybean germplasm. Loci associated with SDS resistance in soybean lines known to be resistant to SDS are used in marker assisted selection during introgression of SDS resistance into elite germplasm. Loci associated with SCN resistance in soybean lines known to be resistant to SCN are used in marker assisted selection during introgression of SCN resistance into elite germplasm. In addition the method may be used to confirm selection of resistance in new soybean cultivars.

The present invention provides a method of introgressing SDS resistance into non-resistant soybean germplasm. Loci associated with SDS resistance in soybean lines known to be SDS resistant are used in marker assisted selection during introgression of SDS resistance into elite soybean germplasm. Examples of soybean germplasm known to be resistant to SDS include Forrest, Pyramid, Essex, Ripley, Jack, Hartwig, PI520.733. P1567507B, P1567.365, P1567.44613 and P1567.373B. The method of the present invention can be used to breed soybeans resistant to any SDS causing F. solani strain.

The method of the present invention comprises the use of nucleic acid markers genetically linked to loci associated with SDS resistance in lines known to be resistant to SDS. The markers are used in genetic mapping of genetic material of soybean lines to be used in and/or which have been developed in a breeding program, allowing for marker assisted selection during introgression of SDS resistance into elite germplasm.

According to the method of the invention, any art-recognized genetic mapping techniques can be utilized, with preferred embodiments utilizing Restriction Fragment Length Polymorphism (RFLP) mapping, AFLP mapping, RAPD mapping or microsatellite mapping, using the nucleic acid markers recognized or applicable to the particular method(s). Markers useful in genetic mapping include, for example, the following: For linkage group G, OI03₄₅₀, OI03₅₁₂, SATT309, SATT214, SATT275, SIUSAT122, CTAAGG280, CGGAGA300, ATGCGA190, AGGCAC310, CCACCA120, CCCTC220, ACGCAT80, OG13₄₉₀, Bng122, SATT163 and SATT38; A112I, OE04₄₅₀, OE02₁₀₀₀, and SATT130. For linkage group N OC01₅₀₀, OO04₁₀₇₅ and SATT9; For linkage group C2, OO05₂₅₀, K455D and OP13₅₀₀; For linkage group B or D, OG01₁₀₀₀, SZ19₅₀₀, and SATT71; or linkage group A2, OW15₁₀₀₀, AO85, OA12₁₀₀₀, BLT65, CCAAGC309, CCCATG349, CCGAAC400, CCGAAC401, CCCATG350, CCAAGC310, OW15500, OD04₅₀₀. For linkage group C1, A063I and SAT40.

An alternative embodiment of the present invention comprises a method of confirming selection for SDS resistance. This embodiment comprises identifying products of a soybean breeding program having in their genetic material the loci associated with resistance to SDS.

A further embodiment of the present invention comprises QTL associated with SDS resistance. Exemplary loci of the invention are mapped (or identified as defined) using particular nucleic acid markers, as discussed above, and are further defined by their association with particular art-recognized linkage groups.

An additional embodiment of the invention comprises soybeans resistant to SDS or to SCN bred according to the method of the present invention, or developed through the identification of parental lines possessing one or more QTL of the invention. Thus, an embodiment of this invention includes soybean plants, seeds and tissue cultures resistant to SDS and to SCN. Further, an embodiment of this invention includes: soybean plants, seeds and tissue cultures that include QTL associated with SOS resistance and with SCN resistance.

An additional embodiment of this invention includes the genetic markers associated with SDS resistance and with SCN resistance that are isolatable from soybeans; and which are free from total genomic DNA. Exemplary markers are set forth more fully below, and are characterized by molecular weight and sequence data. The sequences of particular markers are set forth in FIGS. 3-21. Additionally, the sequences of the markers can be derived from the abbreviations of the markers more fully described below, as these abbreviations include standard PCR primer information and molecular weight information. For the markers of this invention, including the sequence of FIGS. 3-21, it is contemplated that the present invention includes DNA segments from such markers and complementary strands of such markers; DNA sequences which hybridize to the markers or segments thereof; and DNA sequences which but for the degeneracy of the genetic code would hybridize to the markers. More particularly, it is contemplated that DNA segments of the markers such as would be useful as probes for hybridization techniques such as northern blots, southern blots, nuclease protection assay and the like; and that DNA fragments of the markers such as would be useful as PCR primers are within the scope of this invention. The markers are used to close resistance genes, which enables the transfer of such genes among elite cultivars by transformation.

As used herein, the term “DNA segment” refers to a DNA molecule which has been isolated free of total genomic DNA of soybeans. Included within the term “DNA segment”, are DNA segments and smaller fragments of such segments, and also recombinant vectors, including, for example, plasmids, cosmids, phage, viruses, and the like.

The nucleic acid segments of the present invention, regardless of the length of the coding sequence itself, may be combined with other DNA sequences, such as promoters, polyadenylation signals, additional restriction enzyme sites, multiple cloning sites, other coding segments, and the like, such that their overall length may vary considerably. It is therefore contemplated that a nucleic acid fragment of almost any length may be employed, with the total length preferably being limited by the ease of preparation and use in the intended recombinant DNA protocol. For example, nucleic acid fragments may be prepared which include a short stretch complementary to a particular marker, such as, but not limited to any of those in FIGS. 3-21, such as about 10 nucleotides or 25 nucleotides. DNA segments with total lengths of about 1,000, 500, 200, 100 and about 50 base pairs in length are also contemplated to be useful.

Allowing for the degeneracy of the genetic code, sequences which have between 20% and about 50%; or more preferably, between about 50% and about 70%; or even more preferably, between about 70% and about 99%; of nucleotides which are identical to the nucleotides of FIGS. 3-21 will be sequences which are “essentially as set forth in FIG. 3”; “essentially as set forth in FIG. 4”; “essentially as set forth in FIG. 5”; “essentially as set forth in FIG. 6”; “essentially as set forth in FIG. 7”; “essentially as set forth in FIG. 8”; etc. Sequences which are essentially the same as those set forth in FIGS. 3-21 may also be functionally defined as sequences which are capable of hybridizing to a nucleic acid segment containing the complement of FIGS. 3-21 under relatively stringent conditions. Suitable relatively stringent hybridization conditions will be well known to those of skill in the art (Sambrook et al., 1989, Molecular Cloning Laboratory Manual, 2d Edition).

By way of example, nucleic acid hybridization will be affected by such conditions as salt concentration, temperature, or organic solvents, in addition to the base composition, length of the complementary strands, and the number of nucleotide base mismatches between the hybridizing nucleic acids, as will be readily appreciated by those skilled in the art. Stringent temperature conditions will generally include temperatures in excess of 30° C., typically in excess of 37° C., and preferably in excess of 45° C. Stringent salt conditions will ordinarily be less than 1,000 mM, typically less than 500 mM, and preferably less than 200 mM. However, the combinations of parameters is much more important than the measure of any single parameter. (See, e.g., Wetmur & Davidson, 1968; Kanehisa, 1984).

Additionally, now that loci for SDS and SCN resistance have been described herein, it will be apparent to one having ordinary skill in the art that known resistance genes or DNA segments having homology to known resistance genes can be used to identify, confirm and/or screen for SDS or SCN resistance or for loci that confer SDS or SCN resistance. As is known in the art, resistance genes are typically found at common loci on the genome. A probe derived from a known resistance gene is used to prove plant nucleic acids to look for mapping of the probe to the loci for SDS or SCN resistance. Thus, the use of known resistance gene is used to probe plant nucleic acids to look for mapping of the probe to the loci for SDS or SCN resistance. Thus, the use of known resistance genes or DNA segments having homology to known resistance gene to identify, confirm and/or screen for SDS or SCN resistance is contemplated to be within the scope of this invention.

Another embodiment of the invention pertains to an isolated and purified soybean rfs1 gene, said gene capable of conveying Fusarium solani-infection resistance to a non-resistant soybean germplasm, said gene located within a quantitative trait locus mapping to linkage group G and mapped by genetic markers OI03₅₁₂ and Bng122, said gene located along said quantitative trait locus between said markers.

Another embodiment of the invention pertains to an isolated and purified soybean rft1 gene, said gene capable of conveying Fusarium solani toxin resistance to a non-resistant soybean germplasm, said gene located within a quantitative trait locus mapping to linkage group G and mapped by genetic markers OI03₅₁₂ and SATT309, said gene located along said quantitative trait locus between said markers.

Another embodiment of the invention pertains to a method of determining soybean sudden death syndrome resistance in a soybean plant in a greenhouse setting, the method comprising the steps of inoculating, soil with a low density inoculum of Fusarium solani; planting a soybean plant in said inoculated soil; growing said plant in said soil for a predetermined period of time in a greenhouse; isolating Fusarium solani-infected tissue from said plant; culturing said infected tissue for a period of time sufficient to allow for fungal colony forming unit growth; statistically determining disease severity and/or infection severity in said plant using the number of said fungal colony forming units; and comparing said disease severity and/or said infection severity to disease severity and/or infection severity data from genetic markers associated with soybean sudden death syndrome resistance to identify a correlation, the presence of a statisically signficant correlation indicating the presence of soybean sudden death syndrome resistance in said soybean plant.

Another embodiment of the invention pertains to a method of characterizing resistance to soybean sudden death syndrome in a soybean plant, the method comprising the steps of isolating roots from a soybean plant infected by Fusarium solani; culturing the root on a culture plate including a restrictive growth medium that provides for slow fungal growth and restricted bacterial growth; determining root infection severity by statistically evaluating the number of Fusarium solani colony forming units on said culture plate; and characterizing resistance to soybean sudden death syndrome in said soybean plant based on said determined root infection severity.

The invention differs from present technology by enabling selection directly on the resistance alleles of gene by linked markers rather than by the phenotype they cause. It solves the problem quickly and cheaply selecting resistant cultivars, assembling many resistance genes in a single cultivar, mixing resistance genes in novel combinations, defining what genes confer resistance in new cultivars and predicting resistance in cultivars. The invention will be used to improve selection for SDS and SCN resistance in soybean. It enhances the process of soybean breeding for increased yield and disease resistance.

Some of the objects and advantages of the invention having been stated hereinabove, other objects and advantages will become evident as the description proceeds, when taken in connection with the accompanying drawings as best described hereinbelow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A depicts soybean linkage map from the Essex by Forrest RIL population. The map was generated by the computer program Mapmaker using locus segregation data for flower color, 26 RFLPs, 91 RAPD bands, 10 DAF bands and 3 microsatellites. Loci for which linkage was not detected are not shown unless the locus was identified by an RFLP marker that was mapped with the same enzyme and the same band sizes as in Shoemaker and Specht (1995). These anchored RFLP markers were used to identify linkage groups where possible (eg. A); groups with no such markers were numbered arbitrarily (eg. SIU1); or tentatively in parentheses (eg. SIU4 (D, J or K)). Marker loci are identified by a horizontal bar and a probe designation after Cregan et al., (1995). Two subscripts by a RAPD primer name indicates apparently codominant RAPD loci. The most likely position of A85H and Rhg4 between A112H and K636 was inferred from Webb et al., (1995). Distances are given as Haldane centimorgans cM. END indicates the likely position of the telomeres on linkage groups. Thick vertical bars represent regions of the genome less than 10 cM from the nearest marker. Thin vertical bars represent regions of the genome further than 10 cM from the nearest marker.

FIG. 1B shows locations of four QTL conditioning SDS DI and one conditioning SCN race 3 resistance in Forrest and Essex. The QTL were putatively assigned to linkage group C2, G, and N on the soybean genetic map by anchored RFLP (Shoemaker and Specht, 1995; Concibido et al., 1995) or microsatellite markers (Akkaya et al., 1995). END shows the likely position of the telomere on a linkage group. The thin vertical bar indicates the rest of the linkage group. Marker names are given on the left and marker-QTL LOD scores are given on the right of each linkage group. LOD scores at markers were from single-locus analyses of additive gene effects using MAPMAKER/QTL 1.1. Genetic distances (cM) were from the recombinant inbred line function of MAPMAKER/EXP 3.0. A cM distance scale is shown below in group C2. The estimated position of the QTL is shown based on 2 cM interval mapping using MAPMAKER/QTL 1.1. Boxes indicate the region with greater than 1-LOD (10 fold) likelihood intervals.

FIG. 1C depicts exact positions of the SDS and SCN resistance genes on linkage group G relative to SATT309, Bng122D and OI03₅₁₂. To the left is the LOD plot for the RIL population and the supporting map (Chang et al 1996; 1997); to the right is the map from the NIL populations. Markers names for AFLP markers mapped are shown without band size for clarity. The dark bar indicates the confidence interval for the position of the rhg1 and rft1 genes (LOD>5). The grey bars indicate the confidence intervals for the QTL conditioning resistance to SDS leaf scorch (LOD>4). The white bar indicates the confidence interval for the rfs1 gene conditioning root infection severity (LOD>4). Distances are given in centimorgans±0.5 cM.

FIG. 1D depicts exact positions of the SDS and SCN resistance Rhg4 genes on linkage group A2 relative to BLT65, CCAAGC309, CCCATG349, CCGAAC400, CCGAAC401, CCCATG350, CCAAGC310 cM and OW15₅₀₀. Distances are given in centimorgans±0.5 cM.

FIG. 2 shows locations of two QTL conditioning SDS DI and SCN race 3 and race 14 resistance in Pyramid. The QTL were putatively assigned to linkage group A2 and B on the soybean genetic map by anchored RFLP (Shoemaker and Specht, 1995; Concibido et al., 1995) or microsatellite markers (Akkaya et al., 1995). END shows the likely position of the telomere on a linkage group. The thin vertical bar indicates the rest of the linkage group. Marker names are given on the left and marker-QTL LOD scores are given on the right of each linkage group. LOD scores at markers were from single-locus analyses of additive gene effects using MAPMAKER/QTL 1.1. Genetic distances (cM) were from the recombinant inbred line function of MAPMAKER/EXP 3.0. A cM distance scale is shown below group D. The estimated position of the QTL is shown based on 2 cM interval mapping using MAPMAKER/QTL 1.1. Boxes indicate the region with greater than 1-LOD (10 fold) likelihood intervals.

FIGS. 3A and 3B set forth DNA sequence data for markers flanking each major SDS and/or SCN resistance loci.

FIG. 4 sets forth DNA sequence data for markers flanking each major SDS and/or SCN resistance loci.

FIG. 5 sets forth DNA sequence data for markers flanking each major SDS and/or SCN resistance loci.

FIG. 6 sets forth DNA sequence data for markers flanking each major SDS and/or SCN resistance loci.

FIG. 7 sets forth DNA sequence data for markers flanking each major SDS and/or SCN resistance loci.

FIG. 8 sets forth DNA sequence data for markers flanking each major SDS and/or SCN resistance loci.

FIG. 9 set forths DNA Sequence data close to Bng122D determined from bacterial artificial chromosomes. Potential microsattellite sequence are included (bold) and primers used to assay the are underlined.

FIG. 10 sets forth DNA sequence data close to Bng122D determined from Bacterial artificial chromosomes for markers flanking each major SDS and/or SCN resistance loci.

FIG. 11 sets forth DNA sequence data close to Bng122D determined from Bacterial artificial chromosomes for markers flanking each major SDS and/or SCN resistance loci.

FIG. 12 sets forth DNA sequence data close to Bng122D determined from Bacterial artificial chromosomes for markers flanking each major SDS and/or SCN resistance loci.

FIG. 13 sets forth DNA sequence data close to Bng122D determined from Bacterial artificial chromosomes for markers flanking each major SDS and/or SCN resistance loci.

FIG. 14 sets forth DNA sequence data close to Bng122D determined from Bacterial artificial chromosomes for markers flanking each major SDS and/or SCN resistance loci.

FIG. 15 sets forth DNA sequence data close to Bng122D determined from Bacterial artificial chromosomes for markers flanking each major SDS and/or SCN resistance loci.

FIG. 16 sets forth DNA sequence data close to Bng122D determined from Bacterial artificial chromosomes for markers flanking each major SDS and/or SCN resistance loci.

FIG. 17 sets forth DNA sequence data close to Bng122D determined from Bacterial artificial chromosomes for markers flanking each major SDS and/or SCN resistance loci.

FIG. 18 sets forth DNA sequence data close to Bng122D determined from Bacterial artificial chromosomes for markers flanking each major SDS and/or SCN resistance loci.

FIG. 19 sets forth DNA sequence data close to Bng122D determined from Bacterial artificial chromosomes for markers flanking each major SDS and/or SCN resistance loci.

FIG. 20 sets forth DNA sequence data close to Bng122D determined from Bacterial artificial chromosomes for markers flanking each major SDS and/or SCN resistance loci.

FIG. 21 sets forth DNA sequence data close to Bng122D determined from Bacterial artificial chromosomes for markers flanking each major SDS and/or SCN resistance loci.

FIG. 22 sets forth a table of the genetic code.

FIG. 23 sets forth a map of linkage group G.

DETAILED DESCRIPTION OF THE INVENTION

Broadly this invention relates to a soybean plant and a method of producing the same, which is resistant to soybean sudden death syndrome (SDS). This invention relates to the introgression in soybean of genetic material (for the first time identified) which is capable of causing the plant to be resistant to soybean sudden death syndrome (SDS). Additionally the present invention relates to method of introgression of the desired genetic material from one or more parent plants into the progeny with precision and accuracy.

This invention also relates to a soybean plant and a method of producing the same, which is resistant to soybean cyst nematodes (SCN). This invention relates to the introgression in soybean of genetic material (for the first time identified) which is capable of causing the plant to be resistant to soybean cyst nematodes (SCN). Additionally the present invention relates to method of introgression of the desired genetic material from one or more parent plants into the progeny with precision and accuracy.

It should be appreciated that the SDS-resistant or SCN-resistant converted line offers a much improved donor for use in pedigree or backcrossing programs because recombination for genes for yield and other desirable agronomic traits have already been accomplished by the present invention. To assist in the description of this invention the following glossary of terms are provided.

“Converted plant” shall mean any plant having resistance to SDS or resistance to SCN and additionally the plant or an ancestor of the plant was or has been selected by reference to RFLP, RADF, AFLP or microsatellite data for at least one of the loci herein defined.

“Cross-over” shall mean an exchange of segments of homologous chromosomes during meiosis whereby linked genes become recombined; also the product of such an exchange. The cross-over frequency is the proportion of gametes bearing a cross-over between two specific gene loci. It generally ranges from 0 for allelic genes to 50% for genes so far apart that there is always a cross-over between them. The cross-over site is the place in the chromosome where breakage and reunion of DNA strands occur during recombination.

“Introgression” shall mean the entry or introduction of a gene or a quantitative trait loci from one plant into another.

“Introgressing” shall mean entering or introducing a gene or a quantitative trait loci from one plant into another.

“Linkage block” or “linkage group” shall mean an identified chromosomal region containing genetic material that expresses a desired trait.

“Quantitative trait locus” shall mean a genomic region including a gene underlying a trait on which many genes act.

“Recombination” shall mean reassortment of genes or characters in combinations different from what they were in the parents, in the case of linked genes by crossing over.

“An improved soybean plant, and parts thereof” as used herein and in the claims shall mean an entire soybean plant; all parts thereof, including but not limited to seeds, leaves, stems and roots; and soybean plant tissue cultures.

Nucleic acid sequences which are “complementary” are those which are capable of base-pairing according to the standard Watson-Crick complementarity rules. That is, that the larger purines will always base pair with the smaller pyrimidines to form only combinations of Guanine paired with Cytosine (G:C) and Adenine paired with either Thymine (A:T) in the case of DNA or Adenine paired with Uracil (A:U) in the case of RNA.

“Hybridization techniques” refer to molecular biological techniques which involve the binding or hybridization of a probe to complementary sequences in a polynucleotide. Included among these techniques are northern blot analysis, southern blot analysis, nuclease protection assay, etc.

“Hybridization” and “binding” in the context of probes and denatured DNA are used interchangeably. Probes which are hybridized or bound to denatured DNA are aggregated to complementary sequences in the polynucleotide. Whether or not a particular probe remains aggregated with the polynucleotide depends on the degree of complementarity, the length of the probe, and the stringency of the binding conditions. The higher the stringency, the higher must be the degree of complementarity and/or the longer the probe.

“Probe” refers to an oligonucleotide or short fragment of DNA designed to be sufficiently complementary to a sequence in a denatured nucleic acid to be probed and to be bound under selected stringency conditions.

“Detectable moiety” refers to a modification to the probe nucleic acid that enables the experimenter to identify the labeled nucleic acid in the presence of unlabeled nucleic acid. Most commonly, this is the replacement of one or more atoms with radioactive isotopes. However, other detectable moieties include covalently attached chromophores, fluorescent moieties, enzymes, antigens, groups with specific reactivity, chemiluminescent moieties, and electrochemically detectable moieties, etc.

“PCR technique” describes a method of gene amplification which involves sequenced-based hybridization of primers to specific genes within a DNA sample (or library) and subsequent amplification involving multiple rounds of annealing, elongation and denaturation using a heat-stable DNA polymerase.

“RT-PCR” is an abbreviation for reverse transcriptase-polymerase chair reaction. Subjecting mRNA to the reverse transcriptase enzyme results in the production of cDNA which is complementary to the base sequences of the mRNA. Large amounts of selected cDNA can then be produced by means of the polymerase chain reaction which relies on the action of heat-stable DNA polymerase produced by Thermus aquaticus for its amplification action.

“Nuclease protection assay” refers to a method of RNA quantitation which employs strand specific nucleases to identify specific RNAs by detection of duplexes.

“In situ hybridization of RNA” refers to the use of labeled DNA probes employed in conjunction with histological sections on which RNA is present and with which the labeled probe can hybridize allowing an investigator to visualize the location of the specific RNA within the cell.

“Cloning” describes separation and isolation of genes.

“Sequencing” describes the determination of the specific order of nucleic acids in a gene or polynucleotide.

The term “homology” describes a mathematically based comparison of sequence similarities which is used to identify genes or proteins with similar functions or motifs.

Percent similarity may be determined, for example, by comparing sequence information using the GAP computer program, available from the University of Wisconsin Geneticist Computer Group. The GAP program utilizes the alignment method of Needleman et al., 1970, as revised by Smith et al., 1981. Briefly, the GAB program defines similarity as the number of aligned symbols (i.e., nucleotides or amino acids) which are similar, divided by the total number of symbols in the shorter of the two sequences. The preferred default parameters for the GAP program include: (1) a unitary comparison matrix (containing a value of 1 for identities and 0 for non-identities) of nucleotides and the weighted comparison matrix of Gribskov et al., 1986, as described by Schwartz et al., 1979; (2) a penalty of 3.0 for each gap and an additional 0.01 penalty for each symbol and each gap; and (3) no penalty for end gaps.

The term “homology” describes a mathematically based comparison of sequence similarities which is used to identify genes or proteins with similar functions or motifs. Accordingly, the term “homology” is synonymous with the term “similarity” and “percent similarity” as defined above. Thus, the phrases “substantial homology” or “substantial similarity” have similar meanings.

The present invention relates to a novel and useful method for introgressing, in a reliable and predictable manner, SDS resistance into non-resistant soybean germplasm. The method involves the genetic mapping of loci associated with SDS resistance. SDS resistance can be determined in any acceptable manner.

The soybean line selected for mapping is subjected to DNA extraction (Hnetkovsky et al., 1996). Nucleic acid probes are used as markers in mapping the resistance loci, and appropriate probes are selected based upon the mapping method to be used. The probes can be either DNA or RNA probes and mapping is performed using RFLP, RAPD, AFLP, SCAR or microsatellite technology.

In a particular embodiment, DNA probes are used for RFLP, RAPD, AFLP, microsatellite, or SCAR markers. Such probes come from, for example, Pst1-cloned genomic libraries, and the cloned inserts used as probes may be amplified, for example by PCR, LCR, NASBA™, microsatellite amplification using specific primers, SCAR sequence amplification using specific primers, AFLP amplification from EcoRI-MseI digested DNA, or other amplification methods recognized in the art. For example the markers useful in a preferred embodiment of the invention include the following: For linkage group G, OI03₄₅₀, (particularly useful in RAPD) OI03₅₁₂ (RAPD), SATT309 (microsatellite), SATT214 (microsatellite), SATT275 (microsatellite), SIUSAT122 (microsatellite), CTAAGG280 (AFLP), CGGAGA300 (AFLP), ATGCGA190 (AFLP), AGGCAC310 (AFLP), CCACCA120 (AFLP), CCCTC220 (AFLP), ACGCAT80 (AFLP), OG13₄₉₀ (RAPD), Bng122 (RFLP), SATT163 (microsatellite) and SATT38 (microsatellite), A112I (RFLP), OE04₄₅₀ (RAPD), OE02₁₀₀₀ (RAPD), and SATT130 (microsatellite). For linkage group N, OC01₅₀₀ (RAPD), OO04₁₀₇₅ (RAPD), and SATT9 (microsatellite). For linkage group C2, OO05₂₅₀ (RAPD), K455D (RFLP) and OP13₅₀₀ (RAPD). For linkage group B or D, OG01₁₀₀₀ (RAPD), SZ19₅₀₀ (RAPD), and SATT71 (microsatellite); or linkage group A2, OW15₁₀₀₀ (RAPD), AO85 (RFLP), OA12₁₀₀₀ (RAPD), BLT65 (RFLP and SCAR), CCAAGC309 (AFLP), CCCATG349 (AFLP), CCGAAC400 (AFLP), CCGAAC401 (AFLP), CCCATG350 (AFLP), CCAAGC310 (AFLP), OW15₅₀₀ (RAPD), and OD04₅₀₀ (RAPD). For linkage group C1, A063I (RFLP) and SAT40. Of course, it will be apparent to those skilled in the art that other markers that map to loci for SDS resistance may be utilized in the practice of the invention. For RFLP mapping, restriction fragments are generated using specific restriction enzymes, and the digestion, electrophoresis, Southern transfers and nucleic acid hybridizations are conducted according to art recognized techniques. See, e.g., Keim et al., (1989) the disclosures of which are hereby incorporated herein by reference.

In an alternative embodiment of the method of the invention, RAPD technology can be utilized for genetic mapping. A DNA preparation is amplified using art-recognized amplification techniques, and suitable nucleic acid markers are used; for example: For linkage group G, OI03₄₅₀, OI03₅₁₂, OG13₄₉₀, OE04₄₅₀ and OE02₁₀₀₀. For linkage group N, OC01₅₀₀ and OO04₁₀₇₅; For linkage group C2, OO05₂₅₀, and OP13₅₀₀; For linkage group D, OG01₁₀₀₀ and SZ19₅₀₀; for linkage group A2, OW15₁₀₀₀ and OA12₁₀₀₀.

In a soybean breeding program, the method of the present invention envisions the use of marker assisted selection for one or more loci at any stage of population development in a two parent population, multiple parent population, or a backcross population. Such populations are well-known in the art and are described in Fehr W. R. 1987, Breeding Methods for Cultivar Development, in J. R. Wilcox (ed.) Soybeans: Improvement Production and Uses, 2d ed., the disclosures of which are hereby incorporated herein by reference.

Marker-assisted selection according to art-recognized methods may be made, for example, step wise, whereby the different SDS resistance loci are selected in more than one generation; or, as an alternative example, simultaneously, whereby all five loci are selected in the same generation. Marker assisted selection for SDS resistance may be done before, in conjunction with, or after testing and selection for other traits such as seed yield. Marker-assisted selection is generally described in the following U.S. Pat. Nos. 5,536,901, 5,612,191, 5,606,823, 5,574,210, 5,492,547, 5,491,081, 5,476,524, and 5,385,835, the entire contents of each of which are herein incorporated by reference.

The DNA from target populations may be obtained from any plant part, and each DNA sample may represent the genotype of single or multiple plant individuals (including seed).

Marker assisted selection may also be used to confirm previous selection for SDS resistance or susceptibility made by challenging plant with F. solani in the field or greenhouse and scoring the resulting phenotypes.

The following examples are offered by way of illustration and not by way of limitation.

Example 1 Identification of SDS Resistance Loci in Forrest and Essex Materials and Methods Plant Material

The ‘Essex’ by ‘Forrest’ (ExF)F₅ derived population of soybean recombinant inbred lines (RILs) was constructed by crossing Essex (Smith and Camper, 1973) and Forrest (Hartwig and Epps, 1973). Essex is SDS susceptible, while Forrest is SDS resistant (FIG. 1B; Gibson et al., 1994). About 4500 F₂ plants were inbred to the F₅ generation using a single pod descent method. In year one, a random bulk of seed was planted to obtain 500 F₅ plants of which 150 were randomly selected, only intentionally excluding a few agronomically undesirable extremes. In year two, 100 F₅-derived lines of modal maturity (mid-maturity group V) with sufficient seeds for field testing were retained. All inbreeding occurred in fields with minimal incidence of soybean cyst nematode (SCN) and no history of SDS.

SDS Disease Scoring

Eight field experiments were conducted over a four year period encompassing a total of eight locations in southern Illinois, of these useful data could be derived from five locations. The other three were excluded due to poor stand and absent or minimal SDS leaf symptoms resulting from drought. The five were: Villa Ridge Year One (V Year One); Cora Year Two (C Year Two); Pulaski Year Two (P Year Two); Cora Year Three (C Year Three); and Ridgway Year Four (R Year Four) (C Year Two and C Year Three were separate fields, 2 km apart). All fields were selected based on a history of visually uniform SDS infestation and managed as described by Gibson et al., (1994).

The 100 ExF F₅ derived lines were scored for disease intensity (DI), disease severity (DS) and yield as described (Gibson et al., 1994). Two row plots in a partially balanced simple 11×11 lattice design (2 replications, 8 duplications per parent and 5 checks) were used. Disease was rated weekly and the last score before and the first score after R6 (full pod) were used to standardize DI and DS to the R6 stage. DI was defined as the percentage of plants in the plot with visible leaf symptoms. DS was rated as the degree of leaf damage on diseased plants and was scored on a scale of 1 to 9 (1=0-10%/1-5%, 2=10-20%/6-10%, 3=20-40%/10-20%, 4=40-60%/20-40%, 5=>60%/40% chlorosis/necrosis whereas 6=1-33%, 7=34-66%, 8=66-100% premature defoliation and 9=premature death) (Gibson et al., 1994). Yield was determined at harvest from the two row plots trimmed to 3 m (4.3 m planted). There was no intra-plot bordering since the progeny population displayed uniform growth habit and maturity dates. Lattice adjusted means were used whenever the lattice analysis was more efficient than the RCB. To detect transgressive segregants, an LSD (P=0.05) for across environment means of individual lines vs. parents was calculated using the genotype×environment interaction as the error variance.

SCN Score Determination

The methods for SCN scoring of the ExF lines have been described in detail (Matthews et al., 1991; Njiti et al., 1996). Briefly, the lines were characterized for their resistance to SCN race 3 in the greenhouse by their reaction in soil containing a field population of H. glycines collected from Elkville, Ill. in Year One. The race characterization of the soil was based on SCN disease reactions of standard differentials compared with Essex (Rao-Arelli and Anand, 1988). The pots were placed on top of a heating pad to regulate temperature, the temperature in the soil ranged from 20° to 23° C. The index of parasitism (IP) of each F_(5:7) derived line was calculated from the average cyst count of six single-plant replications compared to Essex, 36 days after planting. Using six plant replications allowed the detection of segregation in lines with residual heterogeneity in genomic regions encompassing SCN resistance genes. Lines with a mean IP greater than 8 and less than 25 compared to Essex were screened again but with 18 single plant replications. The group with resistance to SCN (Table 1) was defined as those lines with IP less than 10 (SCN score<1.0). The group of lines defined as susceptible to SCN showed an IP of 10 or greater (SCN score≧1.0). Lines segregating for resistance to SCN were identified by equal numbers of plants within the line with contrasting Ips. SCN IP was converted to a SCN score on a 0-5 scale to ease scoring and reduce within genotype variability associated with the assay method and the unselected field SCN population used. SCN score was defined as 0=0 IP; 1=1-10 IP; 2=11-20 IP; 3=21-60 IP; 4=61-100 IP; 5>100 IP. Trait data were used for QTL analysis both directly (Concibido et al., 1994; Webb et al., 1995) and after being partially normalized by square root transformation.

DNA Clones

Bacterial strains containing cloned soybean PstI genomic DNA inserts were obtained from Dr. R. Shoemaker, USDA ARS, Ames, Iowa (Shoemaker and Specht, 1995). Plant DNA Extraction, Restriction Digestion, Electrophoresis and Southern Transfers

Leaf material for DNA extraction was collected from 20 F_(5:9) field grown plants per genotype in July 1992 (Agronomy Research Center, Southern Illinois University at Carbondale) and frozen (−70° C.).

DNA extraction from leaf tissue was modified for legumes (Hnetkovsky et al., 1996). Aliquots of soybean DNA were digested with one of five restriction enzymes: TagI, DraI, EcoRI, EcoRV, or HindIII based on informative probe enzyme combinations. DNA fragments were resolved by gel electrophoresis on a 10 g L⁻¹ agarose gel, then transferred and fixed to Hybond-N+ (Amersham Corporation, Arlington Heights, Ill.)

The informative probe-enzyme combinations were generated in this study. In total 243 RFLP probes were tested for the ability to detect polymorphism between Essex and Forrest. Polymorphic RFLP loci were referred to using the naming convention suggested by the Soybean Genetics Committee (Soybean Genetics Newsletter, 1995, 22:11-22).

DNA Hybridizations

Plasmid DNA was digested with PstI and inserts were purified by gel electrophoresis and elution for use in radio-labeling reactions as described by Hnetkovsky et al., (1996). Briefly, 50 to 100 ng of probe DNA was labeled with ³²P-dCTP by random hexamer primed synthesis. Hybridization was performed according to techniques described in Hnetkovsky et al., (1996).

RAPD Protocol-Polymerase Chain Reaction

The amplification reactions were performed after Williams et al., (1990) with 180 separate primers from kits A, B, C, D, E, F, G, H and O from Operon Technologies (Almeda, Calif.). The total volume of 25 ml contained 100 mM dATP, dCTP, dGTP, and dTTP, 0.2 mM of a 10 base pair primer, 10-40 ng genomic DNA, and either 1.0 unit tag polymerase with 1.5 mM MgCl₂ (Promega, Madison, Wis.; Perkin Elmer Cetus, Norwalk, Conn.) or 2.5 units of the Stoffel fragment of Tag polymerase with 3 mM MgCl₂. DNA was amplified in a thermal cycle (Savant, New York, N.Y.) programmed for 45 cycles of 1 minute at 94° C., 1 minute at 36° C., and 2 minutes at 72° C. Amplification products were analyzed by electrophoresis on a 14 g L⁻¹ agarose gel stained with ethidium bromide. Primers which generated distinct polymorphic bands between Essex and Forrest across a range of soybean DNA concentration (10-40 ng) were identified and used to amplify DNA samples from the F_(5:9) lines to generate segregation data for mapping analysis. RAPD markers amplified with Tag polymerase reported here were OC01₆₅₀ and OF01₁₀₀₀. All others were amplified by Stoffel fragment polymerase. RAPD markers associated with SDS resistance were amplified independently on three or more separate occasions to assure reproducibility. Polymorphic RAPD loci were referred to using the naming convention suggested by the Soybean Genetics Committee (Soybean Genetics Newsletter, 1995,22:11-22).

Mapping Quantitative Resistance Level

To discover genomic regions associated with SDS resistance the RILs were classified as Essex type or Forrest type (heterozygotes were excluded) for each marker and compared with SDS disease response scores by analysis of variance (ANOVA). One-way ANOVA was performed with SAS (SAS Institute Inc., Cary, N.C.). The probability of association of each marker with each trait was determined and a significant association was declared if P≦0.005 (unless noted otherwise in the text) to minimize the detection of false associations (Lander and Botstein, 1989). Mapmaker-EXP 3.0 (Landers et al., 1987) was used to calculate map distances (cM) between linked markers using the RIL (ri-self) genetic model. To identify intervals containing QTL governing SDS response the marker map and disease data were simultaneously analyzed with Mapmaker/QTL 1.1 (Paterson et al., 1988) using the F₂-backcross model for trait segregation (Webb et al., 1995). Putative QTL were inferred with LOD (log₁₀ of odds ratio) scores exceeded 2.0 at some point in each interval.

Heritability

The broad sense heritability determination of 0.89 for DI indicated the number of plants showing SDS leaf symptoms within inbred lines was highly consistent. With heritability high, non-genetic variation was well controlled and genetic loci contributing to phenotypic variation could be more accurately detected with markers.

Locations of Four SDS Resistance Loci

Four independent marker loci had P values less than 0.005 and LOD scores greater than 2.0 for association with SDS resistance. These markers were located on linkage groups C2, G (two loci) and N (Table 2, FIG. 1). Approximate map position for resistance QTL were estimated based on approximate the size of the marker QTL score and the distances between the markers.

Ten of the 100 lines had greater SDS resistance (a mean DI lower) than Forrest. Eight of these lines were significantly transgressive when the LSD was determined from the genotype×environment of the subset of the 10 lines numerically superior to Forrest and excluding data from Forrest. The smaller LSD₀₅ (4.8) involved in this comparison reflects the consistently low DI scores of these lines in all environments. The eight transgressive lines may be useful starting points for developing soybean cultivars with stronger SDS resistance. Six resistant lines were expected in this inbred population provided 4 genes were needed and had normal 1:1 segregation ratios which is not significantly different from the eight to ten found. One resistant line is to be released to soybean breeders as a germplasm release under the Plant Variety Protection Act (Schmidt et al. 1998).

Lines having resistance marker type on C2, G (both loci) and N were more resistant to SDS than Forrest (Table 3). Both this and the allele trait values for the locus on linkage group C2 showed that resistance to SDS was partly contributed by Essex.

Example 2 Identification of an SDS Resistance Locus in Forrest in the Greenhouse Materials and Methods

Fusarium solani strain ST90, maintained on 5× Bilays medium at 19 C, was transferred onto PDA plates at 28 C for inoculum preparation. The 40 ExF RILs used were the 20 most resistant to SDS in the field and the 20 most susceptible to SDS in the field.

Infested Seed Assay

Plants were inoculated employing the infested oat technique (Stephens et al., 1993). Oats were soaked in tap water overnight, then excess water was strained away. The 150 ml portions were put into 250 ml Erlenmeyer flasks, capped, and autoclaved for 20 minutes. Oats were then cooled and three 1 cm² agar plugs from the colony borders of F. solani petri plates were aseptically added to the sterilized oats. The inoculated oats were incubated in the dark at room temperature (24 C) for three weeks. Flasks were constantly shaken for uniform growth. Plants were grown in a soil medium containing a 1:1 (v/v) mixture of sterilized sand and sterilized soil taken from Ridgway. Inoculation of the plants was done at v2-v3 stage of growth by placing five infested oats next to the taproot below the soil surface. Plants were then irrigated to seal the soil, and soil was kept saturated to ensure the pathogenicity of the fungus.

Cornmeal Sand Assay

The cornmeal method of inoculation was used with a few modifications. Two 1 cm×1 cm square pieces of the infested agar was transferred into a 50 ml volume of cornmeal and silica mix (1:1) and moistened with sterile water. This was kept in an incubator for 14 days. At the same time seeds were sown on steamed sand. After 14 days, the inoculum was mixed with steamed sand/soil (1:1 v/v) in a 1:40 proportion. Styrofoam cups were filled and 14 day old seedlings were transferred into the filled cups. These were set in water filled basins with enough water to keep them moist. The experimental design was a 2-factor randomized complete block with treatment as a split on genotype. After 21 days, disease severity based on a scale of 1-9 (1=plants with no leaf chlorosis and 9=dead plant) was scored. Other trait data were root weight, shoot weight and plant height. Data analysis used a one-way analysis of variance (ANOVA).

Greenhouse Traits

There was a high significant difference between treatments for EXF and the RILs (Table 1). This significant dry root weight and shoot weight confirm the pathogenicity of the strain of Fusarium solani used and the effectiveness of the assays as a whole. As seen on the same table, there was significance among the lines for all the traits measured except disease severity. The frequency distribution of disease severity for the EXF RILs was normal (P>0.16) not bimodal. The field scores and greenhouse scores were not significantly correlated for either assay.

Marker Associations

One marker was associated with the resistance to SDS trait measured for the forty lines tested in the cornmeal sand assay (Table 4). A063I on linkage group C1 was associated with SDS resistance measured by DS. The beneficial allele was from Forrest. Additional analysis using the extreme twenty lines of EXF (10 best and 10 worst for field SDS score) showed OR10₃₈₀ and OR10₄₀₀ on linkage group SIU1 was associated with SDS resistance measured by DS. The beneficial allele was from Forrest.

TABLE 1 Means of disease incidence (DI) and disease severity (DS) of 100 F₅ derived lines from the cross Essex by Forrest n DI (%) DS (1-9) Essex 8 58.9  1.6 Forrest 8 16.5  1.2 Progeny 100 48.5 (23)≠  1.5 (0.3) h (%) 90 78 ≠Standard deviation among progeny line means is shown in parentheses.

TABLE 4 Probabilities of F for variance among lines and treatments Source df DS RWT SHWT HT LINE 41 ns 0.001 0.03 0.05 TREAT 1 0.001 0.001 0.001 0.001 DS—Disease Severity. RWT - Dry root weight. SHWT - Dry Shoot Weight. HT—Height

TABLE 2 Intervals most likely to contain the SDS QTL that show significant associations with mean SDS disease incidence across five locations QTL No. Mean DI % for RILs DNA and with alleles from Marker Linkage group† R² P > F LOD≠ QTL var.§ Essex Forrest OG13₄₉₀ 1G 0.17 0.0001 4.0 17.0 57.8 ± 3.2 38.5 ± 2.9 OI13₄₅₀ 1G 0.20 0.0001 5.1 20.0 60.2 ± 3.2 39.6 ± 3.1 OE04₄₅₀ 2G 0.16 0.0011 3.1 17.4 54.1 ± 3.9 37.6 ± 3.2 OE02₁₀₀₀ 2G 0.10 0.0097 3.7 14.3 48.8 ± 3.6 37.2 ± 2.3 OO05₂₅₀ 1C2 0.13 0.004 2.3 11.1 35.7 ± 3.5 52.5 ± 3.1 K455D-1 1C2 0.16 0.0005 2.9 15.4 40.4 ± 4.4 61.3 ± 3.4 OC01₆₅₀ 1N 0.16 0.001 1.8 10.7 71.3 ± 2.4 49.5 ± 3.8 OF04₁₆₀₀ 1N 0.10 0.009 1.3 8.0 55.7 ± 3.8 41.4 ± 3.5 †C2, G and N were assigned from Soybase, based on Shoemaker and Specht (1995), the SDS QTL were numbered arbitrarily. ≠LOD is the log₁₀ of the odds ratio that supports evidence for the presence of the QTL at the locus from Mapmaker/QTL 1.1. §the percent variation associated with the interval from Mapmaker/QTL 1.1.

TABLE 3 Marker genotypes and SDS DI means of RILs with the most resistant and least resistant phenotypes based on entry means over five locations. SDS Yield A Linkage C Group† G N Line DI kg ha⁻¹ G13≠ I03 O05 K455 E04 E02 C01 F04 GS§ 78* 4.4 4120 − + − + − + + − 3.5 59 7.2 3870 + + + − + + + + 7.0 37 7.8 3850 + + + + + + + + 7.5 44 8.2 3760 + + + − + + + + 6.5 57 8.8 3890 + + + + + + + + 7.5 23 8.9 3960 + + + + − + + + 7.0 67 9.8 3920 + + + + + − + + 6.5 47 10.6 3715 + + + + − − + − 5.0 Forrest 16.5 3760 + + − − + + + + 6.0 Essex 58.9 3330 − − + + − − − − 2.0 †from Soybase, based on Shoemaker and Olson (1993). ≠Presence of the beneficial allele at the marker is +, absence is − and heterogenous or not scored unequivocally is ∘ §Genotypic score is the sum of individual loci based on + = 1, ∘ = 0.5 and − = 0. *Genotype to be released under the Plant Variety Protection Act

TABLE 5 Marker association with traits in the cornmeal sand assay using the extreme 40 ExF lines for field SDS. ALLELIC MEANS MARKER LG TRAIT P R₂ E F A063 C1 DS 0.028 0.429 5.0 3.0 DS—Disease Severity. RWT - Dry root weight. SHWT - Dry Shoot Weight. HT—Height

TABLE 6 Markers showing significant association with field disease parameters as well as greenhouse traits ALLELIC MEANS MARKER LG P(field) TRAIT P R² E F a, Cornmeal Assay OR10₃₈₀ SIU1 0.036 DS 0.010 0.440 3.4 5.1 OR10₄₀₀ SIU1 0.036 DS 0.010 0.440 3.4 5.1 DS—Disease Severity. RWT - Dry root weight. SHWT - Dry Shoot Weight. HT—Height

Example 3 Identification of Two SAS Resistance Genes Close to a SCN Resistance Gene in Forrest Materials and Methods Plant Material

The cross of ‘Essex’ (Smith and Camper, 1973) by ‘Forrest’(Hartwig and Epps, 1973) (ExF) was made and an F₅ derived population of 100 RILs generated. During the studies described herein the RILs were advanced to the F_(5:13) generation from never less than 300 plants per generation. Essex is susceptible to both SDS and SCN, while Forrest is resistant to both SDS and SCN race 3 (Gibson et al. 1994; Hnetkovsky et al. 1996). The potential for unintentional natural selection for resistance to SDS or resistance to SCN was reduced by all inbreeding being carried out in fields with low incidences of soybean cyst nematodes (SCN) and no history of SDS symptoms.

Residual heterogeneity within RILs, theoretically 6.25%, was about 8% as detected by codominant markers at the F_(5:9) (Hnetkovsky et al. 1996; Chang et al. 1996). For the RFLP marker Bng122D six RILs that were heterogeneic were detected. Two of these RILs ExF11 and ExF34 were used to extract subline populations of 40 individuals at the F_(5:9) generation by seed to row descent (Njiti et al. 1997b). Subline populations from the F_(5:9:11) to F_(5:9:13) were used to test for resistance to SDS and SCN.

Assays of Resistance to SDS

The soybean NILs, Essex and Forrest were planted in a randomized complete block design in two row plots, three replications, and four Southern Illinois locations (Villa Ridge Year One (V Year One), Ridgway Year Two (R Year Two), Ullin Year Two (U Year Two), and Ridgway Year Three (R Year Three)). The Ridgway, soil type was Bonnie silt loam, fine-silty mix, acid, mesic Typic Fluvaquents; Villa Ridge soil type was Belknap Silt Loam, coarse-silty, mixed, acid, mesic typic fluquavents, and Ullin soil type was Patton silty-clay loam, fine-silty mix, mesic, Typic Haplaquolls. Experiments were planted between 15 May and 15 June each year. Rows were 0.75 m wide and 3.0 m long, with about 17 plants/m. At Ridgway Year Three we assayed F. solani infection severity (IS) but DX was too low (Essex DX<2.0) to distinguish resistant and susceptible cultivars or NILs. At V Year One, U Year Two and R Year Two we assayed DX where SDS was moderate to severe (Essex DX was 3.0-12; Njiti et al. 1997b).

SDS Disease Scoring

The methods for SDS scoring of the ExF lines have been described in detail (Njiti et al. 1996, Matthews et al. 1991; Hnetkovsky et al. 1996). Three field experiments were conducted during two years in southern Illinois. All fields were selected based on a history of visually uniform SDS incidences. Disease was rated weekly and the last score before and the first score after R6 (full pod) were used to standardize DI (0-100%) and DS (1-9) to the R6 stage. The disease index (DX, 0-100) was calculated as DI.DS/9.

Root Colonization by F. solani

The sample was taken at the R8 (harvest maturity) (Fehr et al. 1971) from Ridgway in Year Three. Following Njiti et al. 1997) five plants per plot were randomly harvested, recovering at least 15 cm of the taproot. Roots from sampled plants were transported on ice to the laboratory where they were stored at 4° C. (1 to 7d) until they were processed for F. solani isolation and quantification.

Based on the finding by Rupe (1989) that the epidermal tissue of the taproot had the highest frequency of F. solani recovery, the isolation in this study was limited to the taproot. A restrictive medium was used that limited fungal growth and restricted bacterial growth (Njiti et al. 1997). The plates were incubated at room temperature for 14 days. Pure colonies of all slow growing fungi from each segment were transferred onto fresh medium, consisting of 24 g/l of PDA and agar, 10 ml/l of 10% NP-10 and 0.064 g/l of tetracycline. The plates were allowed to incubate at room temperature for 14 days.

The percentage of plants yielding blue F. solani from at least one segment was determined as infection frequency (IF). The percentage of segments yielding blue F. solani from all sampled plants of each plot were determined as infection severity (IS).

SCN Index of Parasitism (IP) Determination

Two SCN indexes of parasitism were determined for each population by comparing the number of white female cysts on each genotype to the number of white female cysts on a susceptible check (Rao-Arelli and Anand, 1988). The first index of parasitism (IP1) was determined at Southern Illinois University at Carbondale using a heterogeneous field population of H. glycines, with Essex as the susceptible check. While the second index of parasitism (IP2) was determined at the University of Missouri research center by inoculating the genotypes with 2000±25 eggs from a homogenous isolate of H. glycines, with ‘Hutcheson, (Rao-Arelli, 1994) as the susceptible check. All experiments were done using five single-plant replications per NIL. The mean number of white female cysts on each genotype and the susceptible check were also determined and IP was the ratio of the mean number of cysts on each genotype to the mean number of cysts on the susceptible check.

RFLP Markers

Bacterial strains containing cloned soybean PstI genomic DNA inserts were obtained from Dr. R. Shoemaker, USDA ARS, Ames, Iowa (Shoemaker and Specht, 1995). Polymorphic loci were detected and screened as described by Chang et al. (1996). Polymorphic RFLP loci were referred to after Cregan et al. (1995).

AFLP Markers

AFLP markers were generated from EcoRI MseI digested genomic DNA exactly as described by Vos et al 1996. AFLP markers are represented by the two selective trinucleotides.

Microsatellite Markers

Microsatellite markers were generated and scored by 6% denaturing PAGE exactly as described by Akkaya et al. (1995). All markers used in this study were Bellsville Agricultural Research Center (BARC) markers. Unless otherwise stated all microsatellite markers described within this application are BARC markers.

RAPD Protocol-Polymerase Chain Reaction

The amplification reactions were done after Williams et al. (1990). DNA was amplified as described previously (Hnetkovsky et al. 1996; Chang et al. 1996).

Mapping Resistance Loci

To detect genomic regions associated with SCN IP and resistance to SDS, the RILs were classified as Essex type or Forrest type for each marker. Markers were compared with SDS disease response scores by the F-test in analysis of variance (ANOVA) done with SAS (SAS Institute Inc., Cary, N.C., 1988). The probability of association of each marker with each trait was determined and a significant association was declared if P≦0.05 (unless noted otherwise in the text) since the detection of false associations is reduced in isogenic lines (Lander and Botstein, 1989; Patterson et al. 1990).

Selected pairs of markers were analyzed by the two-way ANOVA using the general linear model (PROC GLM) procedure to detect non-additive interactions between the unlinked QTL (Chang et al. 1996; 1997) or Epistat (Chase et al. 1997). Non additive interactions between markers which were significantly associated with SDS response were excluded when P≧0.05. Selected groups of markers were analyzed by multi-way ANOVA to estimate joint heritabilities for traits associated with multiple QTL. Joint heritability was determined from the R² term for the joint model in multi-way ANOVA.

Mapmaker-EXP 3.0 (Lander et al. 1987) was used to calculate map distances (cM, Haldane units) between linked markers and to construct a linkage map including traits as genes. The RIL (ri-self) and F₃ self genetic models were used. The log₁₀ of the odds ratio (LOD) for grouping markers was set at 2.0, maximum distance was 30 cM. Conflicts were resolved in favor of the highest LOD score after checking the raw data for errors. Marker order within groups was determined by comparing the likelihood of many map orders. A maximum likelihood map was computed with error detection. Trait data were used for QTL analysis (Webb et al. 1995; Chang et al. 1997).

Mean Comparison

The data were subjected to ANOVA (SAS Institute Inc., Cary, N.C.), with mean separation by LSD (Gomez and Gomez 1984). Graphs were constructed by Quattro Pro version 5.0 (Novell Inc., Orem, Utah).

Results

The genomic region identified by the RFLP marker Bng122D, was associated (0.0004≦P≦0.006) with mean SDS DX and IS, accounting for about 16-38% of DX and 38-73% of IS variability (Table 7-8; FIG. 1B). Bng122D was only marginally associated with resistance to SCN. However, the linked (4.3-7.4 cM) microsattelite marker SATT309 was strongly associated with resistance to SCN (0.0001≦P≦0.0003) and explained 24-97% of the variability. SATT309 was also strongly associated with mean DX (0.0001≦P≦0.0003, 25-63% variability) but not IS. Many heterogeneous (11) and recombinant (17) NILs were detected in the populations (n=80) indicating the region contained a locus that selects against fixation (Table 9 and 10). The recombinant NILs among markers and traits enabled fine-mapping of QTL for resistance to SDS and SCN as qualitative loci (FIG. 1B). Our data showed that resistance to SDS DX, SDS IS and SCN IP in Forrest is caused by a two to four gene cluster with close linkage or pleiotrophy between pairs of genes conditioning SDS IS and DX and between SCN IP and SDS DX.

TABLE 7 Markers associated with disease resistance in the ExF34 derived near isogenic line population. Marker Bng122D OIO3₄₅₀ SATT309 Trait P R² Allele ± SE P R² Allele ± SEM P R² Allele ± SEM SCN IP1^(a) 0.050 23% E 0.44 ± 0.19 0.006 30% E 0.55 ± 0.12 0.0001 42% E 0.44 + 0.06 F 0.19 ± 0.18 F 0.24 ± 0.11 F 0.16 + 0.05 SCN IP2^(b) 0.073 19% E 0.89 ± 0.14 0.0001 47% E 1.10 ± 0.06 0.0001 97% E 0.51 + 0.03 F 0.44 ± 0.19 F 0.37 ± 0.14 F 0.02 + 0.02 VYr1DX^(c) 0.046 23% E 8.5 ± 1.40 0.014 21% E 9.5 ± 0.98 0.0005 49% E 9.66 + 0.92 F 4.5 ± 0.98 F 5.90 ± 0.95 F 3.99 + 0.93 UYr2DX 0.003 38% E 8.0 ± 2.05 0.001 35% E 8.73 ± 1.67 0.002 41% E 7.01 + 1.48 F 0.8 ± 0.18 F 2.03 ± 0.83 F 0.91 + 0.42 RYr2DX 0.008 36% E 27.9 ± 4.17 0.0006 37% E 31.2 ± 2.84 0.0001 62% E 30.11 + 2.45 F 12.2 ± 2.54 F 16.9 ± 2.33 F 10.92 + 2.42 MeanDX 0.006 38% E 14.8 ± 2.36 0.0004 38% E 16.5 ± 1.63 0.0001 63% E 15.65 + 1.37 F 5.7 ± 1.12 F 8.29 ± 1.29 F 5.28 + 1.15 IS(RYr3) 0.0004 73% E 56.3 ± 5.1 0.0017 47% E 51.0 ± 4.3 0.0797 8% E 48.43 + 4.93 F 20.9 ± 4.5 F 26. ± 4.9 F 45.80 + 8.80 ^(a)SCN IP1 = SCN index of parasitism, measured in the greenhouse in SIUC and was based on the number of cysts on susceptible check, Essex. ^(b)SCN IP2 was calculated in the same manner as IP1 except that the data was collected after infection with SCN eggs at the University of Missouri research station compared to Hutcheson. ^(c)RYr1, Ridgway Year 1; UYr2, Ullin Year2; RYr2, Ridgeway Year 2. Disease index at R6 stage; DX is DI * DS/9. IS is infection severity by F. solani. E is the allele derived from Essex, F is the allele derived from Forrest.

TABLE 8 Markers associated with Disease resistance in ExF11 sub-lines derived near isogenic line population. Marker Bng122D OIO3₄₅₀ SATT309 Trait P R² Allele ± SEM P R² Allele ± SEM P R² Allele ± SEM SCN IP1^(a) 0.42 3% E 0.40 ± 0.04 0.12 6% E 0.42 ± 0.20 0.003 24% E 0.63 ± 0.24 F 0.24 ± 0.10 F 0.20 ± 0.05 F 0.14 ± 0.02 SCN IP2^(b) 0.02 19% E 0.60 ± 0.12 0.003 21% E 0.62 ± 0.10 0.002 25% E 63 ± 12 F 0.21 ± 0.11 F 0.17 ± 0.09 F 17 ± 11 VYr1DX^(c) 0.03 28% E 6.68 ± 1.06 0.21 9% E 5.56 ± 0.94 0.09 17% E 5.92 ± 1.07 F 3.43 ± 0.86 F 3.69 ± 0.98 F 3.28 ± 0.91 UYr2DX 0.046 16% E 2.19 ± 0.86 0.24 4% E 1.77 ± 0.59 0.03 14% E 2.70 ± 1.01 F 0.41 ± 0.19 F 0.94 ± 0.40 F 0.80 ± 0.35 RYr2DX 0.002 34% E 9.58 ± 1.60 0.11 7% E 7.57 ± 1.24 0.003 27% E 9.47 ± 1.34 F 3.76 ± 0.72 F 4.95 ± 1.02 F 4.20 ± 0.85 Mean DX 0.002 32% E 5.91 ± 0.98 0.09 7% E 4.75 ± 0.77 0.002 25% E 6.11 ± 0.97 F 2.34 ± 0.42 F 3.04 ± 0.60 F 2.64 ± 0.53 Mean IS 0.0039 38% E 43.2 ± 6.40 0.0017 39% E 42.9 ± 5.60 0.183 9% E 35.6 ± 7.6 F 21.1 ± 1.80 F 21.3 ± 3.20 F 25.2 ± 3.4 ^(a)SCN IP1 = SCN index of parasitism, measured in the greenhouse in SIUC and was based on the number of cysts on susceptible check, Essex. ^(b)SCN IP2 was calculated in the same manner as IP1 except that the data was collected after infection with SCN eggs at the University of Missouri research station compared to Hutcheson. ^(c)RYr1, Ridgway Year One; UYr2, Ullin Year Two; RYr2, Ridgeway Year Two. Disease index at R6 stage; DX is DI * DS/9. IS is infection severity by F. solani. E is the allele derived from Essex, F is the allele derived from Forrest.

TABLE 9 Substitution mapping among recombinant lines among ExF 34 derived population either resistant or susceptible based on marker and disease scores. Line SATT309 SCN^(a) SDS^(b) OI03₅₁₂ F. solani ^(b) Bng122D no. Allele IP_1/IP_2 Allele Allele DX Allele IS Allelle Allele 3 H 0.23/0.81 E E 24.8 E 56 E E 9 E 0.77/1.08 E E 14.6 F 31 F F 13 H 0.31/0.56 H E 10.3 E nd nd E 17 E 0.48/1.10 E E 19.5 F 30 F F 19 H 0.34/0.28 H E 15.4 F 19 F E 23 F 0.00/0.02 F F 1.4 F 60 E E 26 H 0.11/0.72 H F 5.9 F 53 E F 28 E 0.50/1.38 E E 11.5 F 36 F F ^(a)An SCN IP1 of less than 0.23 and more than 0.46 were the critical scores used to distinguish between resistant and susceptible lines. An SCN IP2 of less than 0.03 and more than 0.1 were the critical scores used to distinguish between resistant and susceptible lines. Heterogeneous lines (H) were identified by individual plants within a line being resistant or susceptible. ^(b)A DX of 5.9 or less and 10.3 or more were the critical values used to distinguish between resistant and susceptible lines for disease index. ^(c)Infection severity of 36 or less and 54 or more were the critical values used to distinguish between NILs resistant and susceptible to F. solani.

TABLE 10 Sustitution mapping among recombinant lines among ExF 11 derived population either resistant or susceptible based on marker and disease scores. Line SATT309 SCN^(a) SDS^(b) OI03₅₁₂ F. solani ^(b) Bng122D no. Allele IP_1/IP_2 Allele DX Allele Allele IS Allele Allele 17 F 0.11/0.01 F 1.9 F E nd nd H 14 E 0.38/0.26 H 4.1 E F nd nd H 33 F 0.03/0.45 H 3.7 E F nd nd H 28 F 0.18/0.01 F 4.5 E F nd nd H 36 F 0.18/0.00 F 1.2 F F 22 F H 3 H 0.08/0.02 F 1.6 F F nd nd H 1 F 0.08/0.01 F 5.9 E F 50 E E 5 F 0.18/0.75 E 4.9 E F 12 F E 35 F 0.35/1.09 E 2.6 E F 10 F E 39 E 1.11/0.92 E 4.1 E E 23 F F 13 F 0.26/0.01 F 5.3 E F 29 F F 19 E 0.45/0.68 E 2.6 E E nd nd F 18 E 0.30/1.06 E 4.0 E E nd nd F 15 F 0.10/0.02 F 3.0 E E nd nd F 10 F 0.07/0.00 F 1.5 F F 55 E E 16 F 0.11/0.01 F 0.9 F E 56 E E 4 E 0.37/0.60 H 5.6 E E nd nd E ^(a)An SCN IP1 of less than 0.23 and more than 0.46 were the critical scores used to distinguish between resistant and susceptible lines. An SCN IP2 of less than 0.03 and more than 0.1 were the critical scores used to distinguish between resistant and susceptible lines. Heterogeneous lines (H) were identified by individual plants within a line being resistant or susceptible. ^(b)A DX of 1.9 or less and 2.6 or more were the critical values used to distinguish between resistant and susceptible lines for disease index. ^(c)Infection severity of 29 or less and 50 or more were the critical values used to distinguish between NILs resistant and susceptible to F. solani.

Example 4 Identification of SDS Resistance Loci in Pyramid Close to SCN Race 3 Resistance Loci Materials and Methods

Identical to Examples 1 and 3 with the following exceptions.

Plant Material

The Pyramid by Douglas (PxD) F6 derived population of soybean recombinant inbred lines (RILs) is a typical “South by North” cross. Douglas is SDS and SCN susceptible and Pyramid is SDS, SCN race 3 and SCN race 14 resistant. SCN race 14 resistance derives from PI88.788. SSR Protocol-Polymerase Chain Reaction (PCR):

Amplifications were performed in a Perkin-Elmer 9600V thermal cycler after Akkaya et al., (1995). Primers were gifts from Dr. Cregan, USDA-ARS, Beltsville, Md. or purchased from Research Genetics.

Results Heritability

The broad sense heritability determination of 0.8 for DI indicated the number of plants showing SDS leaf symptoms within inbred lines was highly consistent. With heritability high non-genetic variation was well controlled and genetic loci contributing to phenotypic variation could be more accurately detected with markers.

Locations of Four SDS Resistance Loci

Three independent marker loci had P values less than 0.005 and LOD scores greater than 2.0 for association with both SDS resistance and SCN resistance. These markers were located on linkage groups A2, G and D (Table 11, Table 12, FIGS. 2 and 23). We estimated the approximate map position for resistance QTL based on approximate the size of the marker QTL score and the distances between markers.

Gene Pyramiding or Stacking.

The results presented in FIG. 23 (Table 11) show that the SCN resistance genes on linkage groups A2, D and G appear to operate interchangeably. These results imply the genes are functionally the same as one another and that gene product interaction is required for action.

TABLE 11 Markers showing significant (P ≧ 0.005) associations with mean SCN score from Forrest and Pyramid SCN score for RILs QTL with alleles from DNA Marker Linkage group† R² P > F LOD‡ var.§ Essex Forrest a, Forrest OG13₄₉₀ G 0.04 0.056 0.9 4.2 3.4 ± 0.2 2.7 ± 0.3 OI03₄₅₀ G 0.14 0.0001 2.6 12.9 3.8 ± 0.2 2.5 ± 0.3 OW15₄₀₀ A2 0.16 0.0001 2.4 15.1 3.9 ± 0.2 2.3 ± 0.3 BLT65 A2 0.24 0.0001 6.1 25.2 4.2 ± 0.2 2.0 ± 0.3 SCN score for RILs QTL with alleles from DNA Marker Linkage group† R² P > F LOD‡ var.§ Douglas Pyramid b, Pyramid OA12₁₀₀₀ A2 0.17 0.0004 3.1 20.0 1.6 ± 0.2 1.1 ± 0.5 A85H A2 0.09 0.0099 2.0 9.1 2.3 ± 0.2 1.7 ± 0.3 OG01₃₉₀ D 0.14 0.0051 1.2 14.3 1.6 ± 0.3 1.1 ± 0.4 SATT71 D 0.01 0.4 0.3 1.1 1.7 ± 0.2 1.5 ± 0.3 SATT38 G 0.17 0.0005 3.8 17.0 1.7 ± 0.2 1.3 ± 0.2 OD04₉₅₀ G 0.09 0.0097 1.0 10.0 2.0 ± 0.2 2.9 ± 0.3 †G was assigned from Concibido et al., (1995; 1996), based on Shoemaker and Specht (1995). Siu-8 was an anonymous group with no anchored RFLP markers. ? was a single marker not linked to any other locus scored to date. ‡LOD is the log₁₀ of the odds ratio that supports evidence for the presence of the QTL at the locus from Mapmaker/QTL 1.1. nd is not determined because no flanking marker was available. §the percent variation associated with the interval from Mapmaker/QTL 1.1. nd is not determined because no flanking marker was available.

TABLE 12 Intervals most likely to contain the SDS QTL from Pyramid that show significant associations with mean SDS disease index across five locations. Mean DI % for RILs QTL with alleles from DNA Marker Linkage group† R² P > F LOD‡ var.§ Douglas Pyramid B1t65 A2 0.16 0.009 2.0 16.0 53.4 ± 15 37.6 ± 14 A85H A2 0.11 0.0099 2.0 9.1 52.3 ± 12 41.7 ± 10 OG01₃₉₀ D 0.14 0.0051 2.9 14.3 51.6 ± 13 31.1 ± 10 SZ19 D 0.01 0.006 2.0 11.1 51.7 ± 12 33.5 ± 10 SATT38 G 0.17 0.0005 3.8 17.0 51.7 ± 12 31.3 ± 10 SATT309 G 0.24 0.0001 4.4 25.0 55.5 ± 12 29.5 ± 10 OD04₉₅₀ G 0.09 0.0097 1.0 10.0 52.0 ± 12   39 ± 13 †D, G and A2 were assigned from Soybase, based on Shoemaker and Specht (1995), the SDS QTL were numbered arbitrarily. ‡LOD is the log₁₀ of the odds ratio that supports evidence for the presence of the QTL at the locus from Mapmaker/QTL 1.1. §the percent variation associated with the interval from Mapmaker/QTL 1.1.

TABLE 13 DNA markers associated with SDS in single environments. MEAN DI % Allele MARKER TRAITS P n R² Pyramid Douglas SP17₆₅₀ R6DI-PYr1 0.0122 87 0.0716 41.9 ± 23.2 40.8 ± 29.1 A85H R6DI-RYr5 0.0057 33 0.2215 13.1 ± 20   19.1 ± 7.2  R6DI-CYr2 0.05 31 0.1154 34.3 ± 5   56.1 ± 28   R6DIO-4 0.02 31 0.155 51.5 ± 6   1.9 ± 5.1 R6DIO-5 0.02 24 0.1571 52.8 ± 16.8 50.3 ± 17.3 SM11₁₀₀₀ R6DI-CYr1 0.0577 82 0.0442 39.7 ± 11.9 42.0 ± 8.2  SA02₈₀₀ R6DI-PYr1 0.0122 87 0.0716 41.9 ± 23.2 40.8 ± 29.1 SG01₃₉₀ R6DI-RYr5 0.0040 84 0.0965 20.3 ± 17.2 21.3 ± 3.7  R6DI-PYr1 0.02 90 0.0568 43.9 ± 28.4 32.4 ± 13.8 ST16₅₀₀ R6DI-CYr1 0.0294 30 0.1584 30.4 ± 3.7  51.3 ± 14.2 SY13₁₀₀₀ R6DI-CYr2 0.0114 38 0.1650 48.0 ± 25   33.3 ± 14   R6DIO-4 0.02 93 0.1251 54.0 ± 18.3 54.1 ± 11   SY01₇₀₀ R6DI-CYr1 0.0577 82 0.0442 40.0 ± 12.0 42.0 ± 8.2  SP08₉₀₀ R6DI-PYr1 0.0018 88 0.1079 44.5 ± 28.1 37.7 ± 23.6 SX03₁₀₀₀ R6DI-CYr1 0.0216 88 0.0598 37.0 ± 6.9  45.3 ± 23.6 † CYr1, Cora Year One; PYr1, Pulaski Year One; CYr2, Cora Year 2; RYr3, Ridgway Year 3; RYr5, Ridgway Year Five; O-4 four environment mean; O-5 five environment mean.

Example 5 DNA Markers Derived by Sequence Analysis Materials and Methods

DNA bands, such as bands presented in FIGS. 1 and 2, are excised from gels and cloned into plasmids or sequenced directly according to well-known techniques.

Fingerprinting World Soybean Germplasm:

DNA sequence data for markers flanking each major SDS and SCN resistance loci are shown in FIGS. 3-21. This sequence data is used directly for isolating and determining linked sequences by plasmid, cosmid, BAC or YAC cloning. Polymorphisms among SDS and SCN resistant Pis and cultivars and susceptible cultivars are sought. Comparisons to genetic relationships inferred from RFLP and SSR polymorphism data are made. Selection of material for inclusion in breeding programs is thereby improved.

Developing Breeder Friendly Markers:

From the sequence data found in FIGS. 3-21, and from the other markers identified herein, primer pairs, as for example, PCR primer pairs, capable of distinguishing differences among these genotypes are developed. Simple assays for the markers and genes uses a label, such as, but not limited to, a covalently attached chromophores, that do not need electrophoresis are developed to increase the capacity of marker assisted selection to help plant breeders. Non-destructive sampling of dried seed for DNA preparations are developed to allow selection prior to planting. This enables the testing of the effectiveness of marker assisted selection in predicting field resistance to SDS and SCN.

Additionally, now that loci for SDS and SCN resistance have been described herein, it will be apparent to one having ordinary skill in the art that known resistance genes or DNA segments having homology to known resistance genes can be used to identify, confirm and/or screen for SOS or SCN resistance or for loci that confer SDS or SCN resistance. As is known in the art, resistance genes are typically found on at common loci on the genome. A probe derived from a known resistance gene is used to probe plant nucleic acids to look for mapping of the probe to the loci for SDS or SCN resistance. The probe is prepared and mapping is observed using hybridization techniques as described herein and as are known in the art. Thus, fingerprinting world soybean germplasm and developing breeder friendly markers, as described above, are facilitated using known resistance genes or DNA segments having homology to known resistance genes.

Example 6 Soybean Resistant to SDS Selected in Tissue Culture Materials and Methods

G. max L. seeds used to start cultures should be less than six months old and have been stored in darkness at 4° C. Then, the seeds are cultured as follows:

1. Surface disinfect with 70% (v/v) ethanol for 2 min then 20% (v/v) bleach for 20 min. Rinse three times in sterile MS media.

2. Germinate the seed on MS media containing 10 g/l agar, 30 g/l sucrose but no PGRs for 3 d at 27° C.

3. Axenically remove the testa, remove the cotyledonary notes, cut the cotyledons transversely in half and use the distal cotyledonary halves to establish callus cultures.

Callus Initiation

Cotyledonary halves are placed on MS medium with 30 g/l sucrose, 5 mM kinetin, 100 mg/l myoinositol, 0.5 mg/mL thiamine. HCl pH 5.7 at 27° C. unless noted below. The medium contains 5 mM indolebutyric acid as auxin. Place cotyledonary halves in tubes containing 10 mL solidified media. Incubate for 28 d.

Callus Growth Assay

Pieces of callus each approximately 25 mg should be added to sterile tubes containing 10 mL media with varying concentrations of F. solani or extracts thereof. After 28 d at 28° C. the explants are evaluated for growth and growing sectors subcultured.

Suspension Cell Bioassay

Cell suspensions are derived by placing 2 g of a macerated callus in 40 mL of MS medium. The flask, a 125 mL Erlenmeyer flask, should be capped with a foam plug. Subcultures should be made every 14 days into fresh media by allowing the cells to settle, removing the old media by aspiration, adding twice the volume of fresh media and splitting into two flasks.

Soybean Tissue Culture

Soybean tissue capable of regeneration to whole plants are grown in the presence of F. solani or extracts thereof. Cell lines representing mutants capable of continued growth are regenerated and the heritability of SDS determined in these plants or their seed or tissue derived progeny.

Example 7 Marker Assisted Selection and Greenhouse Selection for Resistance to SDS Materials and Methods

The genetic material used included 30 F_(5:10) recombinant inbred lines (RILs) (a subset of a population of 100 lines) from the cross of ‘Essex’ (Smith and Camper, 1973)×‘Forrest’ (Hartwig and Epps, 1973). Essex is susceptible to SDS while Forrest is resistant to SDS (Gibson et al., 1994; Hnetkovsky et al., 1996). The RILs were selected to include three groups (10 RILs per group) based on SDS disease data from five field environments over five years (F_(5:6) to F_(5:10)). The groups were: (1) field SDS-resistant (10 best of the 100 lines); (2) field SDS-moderate (middle 10 of the 100 lines); and (3) field SDS-susceptible (10 worst of the 100 lines). Essex and Forrest were included for comparison.

The F. solani isolate (ST90) used in these assays was isolated from SDS infected roots of the soybean cultivar Spencer in Stonington, Illinois. The isolation process has been described by Stephens et al., (1993). A 1:1 mixture of cornmeal and SiO₂ (SIGMA Chemical Co., St Louis, Mo.) was infested with F. solani (ST90) and allowed to incubate for 10 days after O'Donnell and Gray (1995). Five cm³ of the inoculum were added to 250 ml of setrile water. The average count of spores on ten 1 ml samples, examined on a hemocytometer under a microscope indicated that the initial inoculum density was 4×10⁵ spores per cm³.

Three experiments were conducted in the greenhouse. The first two experiments were designed to evaluate the selected genotypes in the greenhouse for SDS disease severity (DS) using three inoculum densities (high=10⁴, medium=5×10³, and low=3.3×10³ spore per cm³ of soil). The third experiment was designed to evaluate the genotypes in the greenhouse for F. solani root infection severity (IS) using one inoculum density (2×10³ spore per cm³ of soil).

Sterile soil consisting of a 1:1 (v/v) mixture of sterile sand and soil was used in all experiments. Inoculum and soil were mixed in three ratios (v/v) (1:40, 1:80, and 1:120) for the first two experiments; and in one ratio (v/v) (1:200) for the third. All experiments were planted in a randomized complete block design with two replications. Two-week-old seedlings were transplanted into the inoculated mixture in styrofoam cups and kept saturated with water for four weeks. Experiments were conducted between November 1st and March 1st. Plants were grown with a 14-hour photoperiod and the air temperature was from 20-27° C.

The first two experiments were rated for SDS disease severity (DS) at 21 d after inoculation. The DS was rated on a scale of 1 to 9 as described by Njiti et al., (1996). The third experiment was evaluated for IS 21d after inoculation. Roots were collected and each was washed, oven-dried at 45° C. for 48 h, ground using a Wiley cutting mill (Thomas Scientific™, Swedesboro, N.J.), weighed, and added to 2,000 ml of distilled water one ml of the suspension was transferred onto each of five petri plates containing a modified Nash and Snyder's medium consisting of 20 g agar, 15 g peptone, 1 g KH₂PO₄, 0.5 g MgSO₄.7H₂O and 1 L distilled water (Nash and Snyder, 1962). After autoclaving for 40 min and cooling to 45° C. we added, 0.3 g streptomycin sulfate, 0.1 g neomycin, 0.1 g chlortetracycline, 0.05 g rifampicin, and 0.23 g pentachloronitrobenzene (PCNB) (Terrachlor, Uniroyal Chemical Co., Vaugntuk, Conn.). Plates were incubated at room temperature for 14 d. The number of slow-growing fungus colonies with slimy colony morphology and little aerial mycelium (Rupe, 1989) (see arrows in FIG. 1) on each plate was counted as colony forming units (CFU). The CFU per gram of root tissue was calculated and used for analysis.

Because of the wide range, CPU values were square root transformed for all variance components analyses. Data were subjected to analysis of variance (ANOVA) (SAS Institute Inc., Cary, N.C.). Genotype within group by experiment interaction was tested to justify pooling data from the two experiments. Mean comparison by LSD (Gomez and Gomez, 1984). Data were compared against markers that have been previously shown to be associated with SDS resistance in the field by a one way ANOVA (Chang et al., 1996). Correlations were determined using the CORR function of MSTATC (Freed et al., 1990).

Results

The heritability of DS in the greenhouse was 63%, 35%, and 34% for the low, medium and high inoculum densities respectively. IS heritability was 66 percent. The SDS susceptible group means for DS and IS at low inoculum density were significantly higher than those of the moderate and resistant groups (Table 14). DS and IS values from the low inoculum density treatment were significantly correlated with both field DS and disease index (DX) (Table 16). When both greenhouse DS (=<1.9) and IS (=<400) were used as selection criteria for field resistance, all the SDS field susceptible and eight of the 10 field moderate genotypes were eliminated but only four of the 10 partially field resistant genotypes were eliminated (Table 15). These results demonstrate that the combined use of DS and IS values obtained at inoculum densities of less than 3×10³ spore per cm³ of soil is an effective method of selection of soybean genotypes with high resistance to SDS. Both DS and IS from the low inoculum density assays were significantly (P=<0.05) associated with DNA markers that identify major SDS resistance QTLs in the field (Table 17). These results show that a low inoculum greenhouse assay can be used to supplement marker assisted selection for partial field resistance to SDS.

TABLE 14 SDS group mean comparison of F. solani infection severity (IS) as colony forming units (CFU) per gram of soybean roots. SDS group IS mean(CFU/g of tissue) SDS-resistant  400 A† SDS-moderate  598 A SDS-susceptible 1324 B †Means followed by the same letter are not significantly different.

TABLE 15 Comparison of disease severity (DS) and F. solani root infection severity (IS) among all genotypes at low inoculum density in the greenhouse with field DX, DS, and diagnostic DNA markers. SDS Grp Fld† Fld. Fld. GH GH Genotype K455D GH DX DS DS IS OI03450 SATT309 CE021000 (ExF) 78 R R 0.1 1.1 1.8 185 F F E F (ExF) 23 R R 0.5 1.1 1.3 107 F F F E (ExF) 59 R S 0.5 1.2 1.3 466 F F F E (ExF) 67 R S 1.1 1.2 2.8 1369 F F F — (ExF) 57 R R 1.1 1.3 1.9 117 F F F — (ExF) 44 R S 1.1 1.3 2.7 300 — E F F (ExF) 20 R R 1.2 1.2 1.2 249 E F F E (ExF) 47 R S 1.5 1.2 3.0 804 F F E E (ExF) 37 R R 1.9 1.2 1.3 283 F F F E (ExF) 55 R R 3.0 1.1 1.9 73 F F — E Forrest‡ 2.4 1.2 1.4 312 (ExF) 46 M S 5.4 1.4 1.5 615 F F — — (ExF) 14 M S 6.3 1.3 3.4 501 E E F F (ExF) 91 M S 6.4 1.3 1.3 601 — E F — (ExF) 75 M S 6.4 1.4 2.0 202 F F F — (ExF) 49 M S 6.9 1.3 1.8 505 — E F F (ExF) 26 M R 7.2 1.4 1.4 369 F F E E (ExF) 6 M R 8.2 1.5 1.2 199 F F E E (ExF) 73 M S 8.6 1.5 2.2 312 F F F F (ExF) 97 M S 9.1 1.4 2.5 1137 E — E F (ExF) 45 M S 9.7 1.4 1.9 1291 E E F F Essex‡ 12.8 1.7 2.7 1195 (ExF) 7 S S 20.3 2.2 3.2 67 E E E E (ExF) 80 S S 20.2 2.1 3.6 4407 E E — F (ExF) 85 S S 19.8 2.3 4.9 1155 E E — F (ExF) 76 S S 18.7 2.3 3.0 1117 E E E — (ExF) 83 S S 18.7 1.9 3.5 2854 E E — F (ExF) 18 S S 18.6 2.1 4.1 242 E E — F (ExF) 68 S S 18.2 2.1 1.9 2037 E E — F (ExF) 10 S S 17.5 2.0 4.4 574 F E E F (ExF) 51 S S 16.0 2.0 2.5 244 F E E F (ExF) 39 S S 15.7 2.3 3.2 549 E E E E †SDS resistance group (R = SDS-resistant, M = SDS-moderate, and S = SDS-susceptible based on field response. ‡Forrest is resistant and Essex is susceptible to SDS.

TABLE 16 Correlation of field and greenhouse disease parameters (field disease severity (DS), field disease index (DX), greenhouse disease severity, (DS), greenhouse infection severity (IS). Field Field Greenhouse† Greenhouse DS DX DS IS Field DS — 0.95*** 0.70*** 0.39* Field DX — 0.68*** 0.48** Greenhouse DS — 0.37* Greenhouse IS — *, **, ***Significant at 5%, 1% and 0.1% probability levels respectively. †Only the greenhouse DS values from the low inoculum density treatment was used in this table. Correlations values from the other treatments are discussed in the text.

TABLE 17 Associations between DS and IS from low inoculum density in the greenhouse and DNA markers that identify SDS resistance QTLs in the field. Allelic Means R² Prob. Essex Forrest OI03₄₅₀ Greenhouse DS 24 0.0091 3.1 ± 0.3 2.1 ± 0.2 G-house IS 20 0.0181 1315 ± 398  441 ± 94  Field DS 40 0.0004 1.9 ± 0.1 1.4 ± 0.1 Field DX 42 0.0003 15.2 ± 2.0  5.5 ± 1.3 SATT309 Greenhouse DS 38 0.0004 3.1 ± 0.3 1.8 ± 0.2 Greenhouse IS 18 0.0210 1096 ± 304  382 ± 95  Field DS 49 0.0001 1.9 ± 0.1  1.3 ± 0.03 Field DX 56 0.0001 14.3 ± 1.6  3.3 ± 0.8 OE02₁₀₀₀ Greenhouse DS 17 0.0531 2.7 ± 0.3 1.9 ± 0.2 Greenhouse IS 00 0.8170 524 ± 121 485 ± 112 Field DS 36 0.0026 1.7 ± 0.1 1.3 ± 0.0 Field DX 34 0.0035 11.4 ± 2.2  4.0 ± 0.9 K455D Greenhouse DS 23 0.0229 1.9 ± 0.3 3.0 ± 0.3 Greenhouse IS 16 0.0527 316 ± 74  1124 ± 331  Field DS 09 0.1502 1.4 ± 0.1 1.7 ± 0.1 Field DX 17 0.0439 6.0 ± 2.2 12.2 ± 1.9 

Example 8 Marker Assisted Selection for SDS Resistance in the Field Materials and Methods Genetic Material

Year One Cross between soybean cultivars Flyer, susceptible to SDS and SCN & Hartwig, resistant to both. Year Two F₂ seeds generated were used to produce F₃ seeds by single pod decent. Year Three Advanced to F₅ in the winter nursery in Puerto Rico Year Four 786 individual F₅ plants were randomly harvested as RILs. Year Six 400 lines grown at ARC to select lines - 94 sent to Puerto Rico for increased seed supply Year Seven F_(5:6) seed were planted in greenhouse and leaf samples were used for analysis on all 786 and F_(5:8) seeds on 50 recombinant inbred lines planted in field for SDS rating.

Marker Methodology

Approximately 100 ng (enough for 20 reactions) of PCR ready DNA was extracted from 50 mg of fresh young leaf tissue using a Matrix Mill. The Matrix Mill uses a 30 second burst of an electromagnetically driven metal pestle. BLT65 was amplified using SCAR primers and was visualized on 1.4% agarose gel. SATT38 was amplified using microsatellite primers and was visualized on high resolution FMC agasrose gel. BLT65 was scored as S=single band and R=double band. SATT38 was scored as S, with about 10% more bp., as being slightly higher than R.

Two Markers Used to Identify Four Groups

Marker 1 Marker 2 Class N Group 1 pBLT65(H) SATT38(H) R/R 12 Group 2 pBLT65(H) SATT38(F) R/S 11 Group 3 pBLT65(F) SATT38(H) S/R 9 Group 4 pBLT65(F) SATT38(F) S/S 18 pBLT65 Maps close to SCN resistant gene Rhg4 SATT38 Maps close to the SDS resistant rfs1, rft1 and SCN resistant gene rhg1

Field Test

Two locations, Ridgway and Ullin, were selected for this study based on a history of uniform SDS symptoms. Genotypes were planted in a randomized complete block design in 2 row plots in 2 replications. Data was collected twice from each location at R6—only 3 plants due to yield concerns and R8—five plants. Roots were transported in a cooler to the laboratory where they were stored at 4° C. (1-7 days) until they were processed for F. solani isolation and quantification.

Restrictive Medium

A restrictive medium was prepared as follows:

960 mL of distilled water 0.012 g/mL potatodextrose agar (PDA) 0.012 g/mL agar 10 mL/L of 10% (v/v) tergitol NP-10 Autoclaved and cooled to about 6° C. Fungistatic Antibiotics—limited fungal growth 10 mL/L of 0.025 g/mL penta-chloro-nitro-benzene [PCMB, 75% (w/v)] 10 mL/L of 0.002 g/mL of Botran Bacteriocidal Antibiotics—restricted bacterial growth 10 mL/L of 0.012 g/mL of tetracycline 10 mL/L of 0.012 g/mL of neomyucin sulfate 10 mL/L of 0.012 g/mL of streptomyucin 10 mL/L of 0.012 g/mL of rifampicin (2nd sampling only)

Thus, by the term “restrictive medium” it is meant a growth medium that allows for limited fungal growth or slow fungal growth, while at the same time substantially restricting and/or preventing bacterial growth.

Root Preparation

Roots were cut and lateral roots were removed. The lateral roots were then washed to remove all soil & rinsed in distilled water. The root surface was sterilized in 100 mL/L NaC10 for 3 minutes. The roots were then dipped in dulute (40 μg/mL) tetracycline solution (sterile conditions). Ten 1″ segments were randomly selected and placed on the restrictive medium. Each plant was treated separately, then incubated at room temperature for 14 days.

Quantification

F. solani is slow growing and stains PDA medium blue. For each plate, we counted the number of slow-growing blue fungi. This number was expressed as a percentage of the total number of pieces. The percentage of segments yielding blue F. solani from all sampled plants of each plot were determined as infection severity (IS). The data were subjected to ANOVA, with the mean separation by LSD.

Results

At Ullin, higher infection severity maybe due to wet conditions immediately after planting. At Ridgway, low infection severity maybe due to late planting and dry conditions immediately after planting. Leaf scorch was not measurable (DX<1) at either location, possibly due to a combination of late planting and dry conditions during the reproductive period. The increased sensitivity of IS evaluation by addition of rifampicin in selective medium increased the number of Fusarium solani colony forming units on second sampling.

We found SATT38 to be effective in selecting soybean cultivars with resistance to infection by F. solani. The use of pBLT65 in addition to SATT38 for MAS did not improve resistance to infection compared to the use of SATT38 alone. Both SATT38 and pBLT65 were required for SCN resistance to be present indicating digenetic inheritance.

In summary, allelles or genes conferring partial resistance to SDS in E×F function to confer resistance across crosses, across resistant cultivars, across genetic backgrounds, across growth habits, and across maturity groups. Allelles or genes conferring resistance to SCN in E×F function to confer resistance across crosses, across resistant cultivars, across genetic backgrounds, across growth habits, and across maturity groups.

TABLE 18 ANOVA for markers selection ULLIN RIDGWAY Source df MS P > F MS P > F Sample 1 Rep 1 2057 0.0100 109 0.3892 Grp 3 232 0.4921 237 0.1925 Gen(Grp) 46 422 0.0927 151 0.4353 SDS-R v SDS-S 1 440 0.2201 109 0.3890 Grp x Rep 3 438 0.2171 354 0.0750 Error 46 285 144 Sample 2 Rep 1 486 0.1899 208 0.3619 Grp 3 2874 0.0001 508 0.1178 Gen(Grp) 46 380 0.0836 324 0.1764 SDS-R v SDS-S 1 8021 0.0001 1430 0.0199 Grp x Rep 3 277 0.3544 246 0.9920 Error 46 249 8

TABLE 19 Marker assisted selection for SDS and SCN in Hartwig by Flyer derived recombinant inbred lines. Numbers in columns followed by letters that differ indicate means that are significantly different. % Infection Severity Marker 1 Marker 2 Class N Ullin Ridgway Mean SCN score Group 1 pBLT65(H) SATT38(H) R/R 12 34a 25a 29a 2.03a Group 2 pBLT65(H) SATT38(F) R/S 11 47b 24a 35b 3.74b Group 3 pBLT65(F) SATT38(H) S/R 9 35a 21a 28a 3.16b Group 4 pBLT65(F) SATT38(F) S/S 18 48b 27a 37b 2.99b

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It will be understood that various details of the invention may be changed without departing from the scope of the invention. Furthermore, the foregoing description is for the purpose of illustration only, and not for the purpose of limitation—the invention being defined by the claims. 

1. A method of conveying soybean sudden death resistance into non-resistant soybean germplasm comprising the step of introgressing soybean sudden death resistance into non-resistant soybean germplasm using one or more nucleic acid markers for marker assisted selection among soybean lines to be used in a soybean breeding programs, said markers associated with soybean sudden death syndrome resistance.
 2. (canceled)
 3. The method of claim 1, wherein said marker assisted selection comprises the use of an analysis technique selected from the group consisting of RAPD analysis, RFLP analysis, microsatellite analysis, and AFLP analysis.
 4. The method of claim 1, wherein the source of the sudden death syndrome resistance comprises a soybean strain selected from the group consisting of Forrest or a descendant thereof, Essex or a descendant thereof, Pyramid or a descendant thereof, Ripley or a descendant thereof, Jack or a descendant thereof, PI520733 or a descendant thereof, PI567507B or a descendant thereof, PI567.365 or a descendant thereof, PI567.446B or a descendant thereof, PI567.373B or a descendant thereof, PI90763 or a descendant thereof, and PI209332 or a descendant thereof. 5-7. (canceled)
 8. A quantitative trait locus associated with resistance to soybean sudden death syndrome, said locus mapping to a linkage group in the soybean genome, said linkage group selected from the group consisting of G, N, C2, B, D, A2, and C1.
 9. A quantitative trait locus of claim 8, said locus mapping to linkage group G and mapped by one or more of the markers OI03₄₅₀, OI03₅₁₂, SATT309, SATT214, SATT275, SIUSAT122, CTAAGG280, CGGAGA300, ATGCGA190, AGGCAC310, CCACCA120, CCCTC220, ACGCAT80, OG13₄₉₀, Bng122, SATT163, SATT38, A112I, OE04₄₅₀, OE02₁₀₀₀, and SATT130.
 10. A quantitative trait locus of claim 8, said locus mapping to linkage group N and mapped by one or more of the markers OC01₅₀₀, OO04₁₀₇₅ and SATT9.
 11. A quantitative trait locus of claim 8, said locus mapping to linkage group C2 and mapped by one or more of the markers OO05₂₅₀, K455D and OP13₅₀₀.
 12. A quantitative trait locus of claim 8, said locus mapped by one or more of the markers OG01₁₀₀₀, SZ19₅₀₀ and SATT71.
 13. A quantitative trait locus of claim 8, said locus mapping to linkage group A2 and mapped by one or more of the markers OW15₁₀₀₀, AO85, OA12₁₀₀₀, BLT65, CCAAGC309, CCCATG349, CCGAAC400, CCGAAC401, CCCATG350, CCAAGC310, OW 15500, and OD04₅₀₀.
 14. A quantitative trait locus of claim 8, said locus mapping to linkage group C1 and mapped by one or more of the markers A063I and SAT42. 15-21. (canceled)
 22. An improved soybean plant, seed or tissue culture produced by the method of claim
 1. 23. (canceled)
 24. An improved soybean plant, and parts thereof, which evidences a resistance response to soybean sudden death syndrome (SDS) comprising a genome, homozygous with respect to genetic alleles which are native to a first parent and normative to a second parent of the improved soybean plant, wherein said second parent evidences significantly less resistant response to SDS than said first parent and said improved plant comprises alleles from said first parent that evidences resistance to SDS in hybrid combination in at least one locus selected from: a locus mapping to linkage group G and mapped by one or more of the markers OI03₄₅₀, OG13₄₉₀, Bng122, SATT163, SATT38, A112I, OE04_(450, OE)02₁₀₀₀ and SATT130; a locus mapping to linkage group N and mapped by one or more of the markers OC01₅₀₀, OO04₁₀₇₅ and SATT9; a locus mapping to linkage group C2 and mapped by one or more of the markers OO05₂₅₀, K455D and OP13₅₀₀; a locus mapped by one or more of the markers OG01₁₀₀₀, SZ19₅₀₀ and SATT71; a locus mapping to linkage group A2 and mapped by one or more of the markers OW15₁₀₀₀, A085 and OA12₁₀₀₀; or a locus mapping to linkage group C1 and mapped by one or more of the markers A063I and SAT42, said resistance not significantly less than that of the first parent in the same hybrid combination, and yield characteristics which are not significantly different than those of the second parent in the same hybrid combination.
 25. The inbred soybean plant of claim 24 comprising each of: a locus mapping to linkage group G and mapped by one or more of the markers OI03₄₅₀, OG13₄₉₀, Bng122, SATT163, SATT38, A1121, OE04₄₅₀, OE02₁₀₀₀ and SATT130; a locus mapping to linkage group N and mapped by one or more of the markers OC01₅₀₀, OO04₁₀₇₅ and SATT9; a locus mapping to linkage group C2 and mapped by one or more of the markers OO05₂₅₀, K455D and OP13₅₀₀; a locus mapping to linkage group D and mapped by one or more of the markers OG01₁₀₀₀, SZ19₅₀₀ and SATT71; a locus mapping to linkage group A2 and mapped by one or more of the markers OW15₁₀₀₀, A085 and OA12₁₀₀₀; or a locus mapping to linkage group C1 and mapped by one or more of the markers A063I and SAT42, and having improved resistance to SDS when compared to a substantially identical inbred soybean plant not comprising said loci.
 26. A soybean plant, and parts thereof, as claimed in claim 24 comprising the progeny of a cross between first and second inbred lines, alleles conferring resistance to sudden death syndrome (SDS) being present in the homozygous state in the genome of one or the other or both of said first and second inbred lines such that the genome of said first and second inbreds together donate to the hybrid a complement of alleles necessary to confer the resistance to SDS.
 27. A sudden death syndrome resistant hybrid and parts thereof formed with the improved soybean plant according to claim
 25. 28. A soybean plant and parts thereof formed by selfing the sudden death syndrome resistant hybrid of claim
 27. 29. (canceled)
 30. A method for identifying soybean sudden death syndrome (SDS) resistance or soybean cyst nematode resistance in a soybean plant using a SDS resistance gene, a SCN resistance gene or DNA segments having homology to a SDS resistance gene or to a SCN resistance gene, the method comprising the steps of: (a) probing nucleic acids obtained from the soybean plant with a probe derived from said SDS resistance gene or from said SCN resistance gene or from said DNA segment having homology to said SDS resistance gene or to said SCN resistance gene; and (b) observing hybridization of said probe to said nulceic acids, the presence of said hybrization indicating SDS or SCN resistance in said soybean plant.
 31. The method of claim 30, further comprising using the probe derived from a known resistance gene to probe plant nucleic acids to look for mapping of the probe to loci for SDS or SCN resistance, the presence of said mapping indicating the presence of SDS or SCN resistance.
 32. (canceled)
 33. A quantitative trait locus associated with resistance to soybean cyst nematode, said locus mapping to linkage group B and mapped by one or more of the markers OG01₁₀₀₀, SZ19₅₀₀ and SATT71.
 34. (canceled)
 35. An improved soybean plant having soybean cyst nematode (SCN) resistance, the genome of which contains genes associated with said SCN resistance at a locus mapping to linkage group B and mapped by one or more of the markers OG01₁₀₀₀, SZ19₅₀₀ and SATT71.
 36. An isolated and purified soybean rfs1 gene, said gene capable of conveying Fusarium solani-infection resistance to a non-resistant soybean germplasm, said gene located within a quantitative trait locus mapping to linkage group G and mapped by genetic markers OI03₅₁₂ and Bng122, said gene located along said quantitative trait locus between said markers.
 37. An isolated and purified soybean rft1 gene, said gene capable of conveying Fusarium solani toxin resistance to a non-resistant soybean germplasm, said gene located within a quantitative trait locus mapping to linkage group G and mapped by genetic markers OI03₅₁₂ and SATT309, said gene located along said quantitative trait locus between said markers.
 38. A method of determining soybean sudden death syndrome resistance in a soybean plant in a greenhouse setting, the method comprising the steps of: (a) inoculating soil with a low density inoculum of Fusarium solani; (b) planting a soybean plant in said inoculated soil; (c) growing said plant in said soil for a predetermined period of time in a greenhouse; (d) isolating Fusarium solani-infected tissue from said plant; (e) culturing said infected tissue for a period of time sufficient to allow for fungal colony forming unit growth; (f) statistically determining disease severity and/or infection severity in said plant using the number of said fungal colony forming units; and (g) comparing said disease severity and/or said infection severity to disease severity and/or infection severity data from genetic markers associated with soybean sudden death syndrome resistance to identify a correlation, the presence of a statistically significant correlation indicating the presence of soybean sudden death syndrome resistance in said soybean plant.
 39. The method of claim 38, wherein said low density inoculum of Fusarium solani comprises an inoculum density of about 3×10³ spore/cm³ soil.
 40. The method of claim 38, wherein said Fusarium solani-infected plant tissue comprises root tissue. 41-42. (canceled) 